Before you read this, bear in mind how huge my post is :)
A couple of weeks ago, I started using Baton Pass just to try it in 6th Gen, having only ever played it in 3rd Gen OU. As people may or may not know, Baton Pass was a divisive topic in ADV as well - I wasn't around 10 years ago but a friend told me about this. I believe that in 3rd Gen, Ingrain Smeargle + Baton Pass was banned.
This generation, i swiped a team from somewhere and tried it out. Initially i did this as a joke to screw with 2 friends of mine, but then I actually tailored the sets to make as good a team as possible: I ended up with Scolipede, Sylveon, Mawile (Mega), Vaporeon, Espeon and Smeargle. Using this team on the ladder, I came up against AJTheEpic, who had one of the supposed roadblocks to Baton Pass in Prankster Thundurus with Taunt. Despite him having this and me making a nasty misplay or two, which actually ended up breaking the chain, I was able to retreat and start the chain again, eventually beating him.
After the match, we shortly discussed Baton Pass as a strategy and he said something which i believe to be very relevant - his words were "I have no way to predict against your team". Obviously prediction is a large part of the game and rewards better players who make smart plays and win games because of this, but when playing Baton Pass, this goes out of the window.
The biggest issue with the chains is the Speed accumulation which allows you to spam Baton Pass and always stay a step ahead of your opponent. Scolipede has risen as the lead member of the chain, passing +2 speed to its teammates. Obviously things like Vaporeon and Sylveon are still slow, but I worked a way around this: you can invest certain amounts of speed into each of your teammembers so that they will outspeed relevant speed tiers with a certain number of speed boosts: for example, i decided to invest enough speed into Mawile so that it would outspeed the base 108 and 110 speed tiers with +2 speed, so that it could come in, kill/scare off certain threats in that speed range, and then continue the chain.
I do not claim do be an expert, but Baton Pass has always interested me as a playstyle, and I've tried it in almost every tier in every Generation. In Generation V, Baton Pass obtained two very useful tools in Magic Bounce Espeon and Stored Power, which help(ed) BP's viability hugely. combine this with the lack of shitty boring HO weather offense slower pace of OU this generation (thank you Defog), and BP is suddenly a lot better than it was last generation, although i believe that it is nowhere near as good as it was/is in ADV.
Something I've seen several inexperienced players try to do (kudos to them btw, lots of people higher up on the ladder just forfeit as soon as they see team preview) is stack hazards on my side of the field, as it will then restrict my switching. this is a REALLY bad idea because: a. i can just pass into Espeon and bounce all their hazards back at them, and b. the best way of winning against Baton Pass chains is to stop them before they start to snowball and accumulate their boosts - laying hazards whilst Vaporeon sets up to +6 with Acid Amor is a really bad idea, to think of one match i had - and then I just quickly passed back into Scolipede to absorb the Toxic Spikes, then back out into Espeon to bounce them back, then i just set up CM a couple of times and won 6-0 with Stored Power.
A note on Stored Power: a mistake I have seen several players make is to try and accumulate +6 in every stat, especially Special Attack, before going on a rampage with Stored Power. after +3 Speed, +4 Defense and +4 Special Defense (for example), Stored Power becomes stupidly powerful, able to do things such as kill Shield Forme Aegislash from full through its Psychic Resistance. Too many players try to get all the boosts possible, where they should actually be focusing on passing Espeon the speed it needs, as well as the defensive boosts it needs, so that she can take several priority hits and click Stored Power six times to win.
But what about Dark Types, you ask? well, Espeon gets this nice Fairy Move to use purely for Dark-Types that are immune to Stored Power. However, this is not the only way to deal with Bisharp/Tyranitar trying to kill Espeon. Baton Pass has a nice side effect of allowing her to escape Pursuit unharmed (a mechanic which quite a few players around the 1650 ranking apparently don't know about). I personally run Sylveon with Hyper Voice/Calm Mind/Substitute/Baton Pass in my chain, as she can get rid of things that try to set up Substitute as you set up the chain, as well as being able to come into Pursuits and scare away any Dark Types with Hyper Voice. Oh, and sometimes I run Stored Power on Sylveon too, just in case.
Let's think of some other Baton Passers now. You can tailor your chain to deal with certain things - I run Vaporeon with that little bit of Speed investment, then throw the rest into HP and Defense. A typical Vaporeon set in a full BP Chain will be Scald, Substitute, Acid Armor and Baton Pass - this is what I use, and I imagine it is the standard. with Calm Mind boosts, Vaporeon can pose a significant threat to anything with Scald. Also, alongside the burn chance and Acid Armor, Vaporeon becomes a monstrous physical wall.
Now let's talk about Smeargle - he can take the pressure off Espeon/Mr Mime immensely with Ingrain, meaning that the Psychic types don't have to come into onslaughts from opponents that expect their switchin and hammer them with attacks instead. Smeargle can also Spore pretty much anything after he receives a Substitute and some Speed, and can pass any boost you like - I've seen interesting setups of Baton Pass which involved Speed, Defense and Special Defense accumulation, then a safe pass into Smeargle (who was protected by Sub of course). Smeargle then proceeded to Smash on the predicted Grass-Type switchin, then put it to sleep with Dark Void, then Smashed again, then passed a Vaporeon sub and all those boosts to Espeon, and it was gg at that point).
Let's think of some supposed answers to full Baton Pass now:
Unaware Clefable: She can set up Calm Mind or Cosmic Power and proceed to hammer you with attacks, who can just sit there and wall you. However, no amount of boosts will save Clefable from base 600 Stored Power coming from Espeon. However, there is another easy way of dealing with Clefable - Steel Types with Baton Pass. Scizor or Mega Mawile work nicely here, being able to break Clefable with STAB Steel moves - personally, I run Mega Mawile with Iron Head. These Steel-Types can also pass SD/Iron Defense, so they can contribute to the chain outside of getting rid of the Mt. Moon fairy.
Unaware Quagsire - cannot take base 500 or whatever Stored Power. If it has Haze, it can singlehandedly save its team - again, AJTheEpic's Terrible Trireme team has saved me from Baton Pass chains, as he runs Haze on his Quagsire - however, this isn't foolproof, as my friend found out when trialling AJ's Triremes team against a Baton Pass team of mine - Celebi is more than viable in Baton Pass chains, and with smart play and liberal used of dry Passing, you can easily stop Quagsire from Hazing, and set up with Celebi as it switches out. I have also run Hidden Power Grass on Zapdos before for this very roadblock to the team.
powerful attackers such as Talonflame/Mega Pinsir: Scolipede has a nasty disadvantage against these two, but there are ways around this too. Agility Zapdos can scare both of them away and set up nice and easily. Mega Mawile can come in on both (if Talonflame is locked into Brave Bird), and can easily Intimidate them, Mega Evolve and set up Acid Armor.
Hyper Voice Sylveon/Mega Gardevoir - Specs Sylveon is grim to play against, as its Hyper Voices are stupidly powerful. However, you can run Poison Jab on Scolipede to deal with these. Taunt Deoxys is annoying, but you can run Mental Herb on Scolipede to deal with this (Protect first turn, then Sub on the Taunt which Mental Herb cures, then pass into Espeon). you can also run Megahorn to kill off Deoxys-S - oh, and Scolipede outspeeds Deoxys-S after one Protect, which also helps you scout for Psycho Boost).
basically, what i wanted to say is that all or most of the supposed "counters" can be dealt with in some form or another. Taunt Sableye is easy to predict, and you can just have a war with it with something like Vaporeon (repeated Scalds, whilst it can't Recover lest Taunt wears off), or simply kill it with Sylveon. certain passers such as Gliscor and Mr Mime (i think) can Taunt opposing Taunters, and there's always Mental Herb. Things like Mega Pinsir can be dealt with by specialising certain teammembers, eg Agility Zapdos.
Another couple of points I would like to mention is that Baton Pass chains can really really fuck with several prominent OU tier members. Let's take Aegislash for example. It can threaten Espeon with Ghost moves such as Shadow Sneak and Shadow Ball. However, you can Pass into something like Vaporeon, which can set up Substitutes and Acid Armors on Aegislash. Also, Aegislash is lefft in a nasty situation after having attempted to kill Espeon - it is now in Sword Form, which leaves it very vulnerable to getting wrecked by Scald from Vaporeon. Does it keep attacking as Vaporeon sets up on it, King's Shield as Vaporeon sets up on it, or switch out and give Vaporeon a free turn to set up? you can also be a dick and bring Smeargle into Ghost moves, threaten Aegislash with Spore, then use the free turns to set up even more, or simply dry pass to see if/what Aegis switches out into.
Another example is Mandibuzz. Mandibuzz comes in on, say, Scolipede. Now, it can Whirlwind, but you're probably going to pass into Espeon, right? so Mandibuzz goes for the Foul Play/Brave Bird/Knock Off. Interestingly, Espeon's Substitutes aren't broken by any of the mentioned moves if an Acid Armor has been passed to it. So Mandibuzz can hit Espeon's Subs as Espeon Calm Minds. then Espeon passes into Sylveon on a Foul Play. If the Baton Pass player is smart, he will manage the damage taken so that Sylveon can come in on Mandibuzz and have a Sub intact at the end of the turn. Mandibuzz can then either die to Hyper Voice or switch. Free turns again.
A final example is Heatran - let's say it comes in on Mawile. Mawile will be faster due to the Speed boosts. This situation is slightly more difficult - pass ino Vaporeon or Espeon? passing into Espeon is the best option, as you can bounce Roar back at Heatran, forcing it out, then either pass out of the dragged-out Pokemon or set up on it. Or you can set up Calm Mind on Heatran if you like.
To sum up this huge wall of text, Baton Pass is difficult to play against at the best of times. As much as i have loved the strategy over the years, it is not fair that I was able to beat AJTheEpic with a kindof halfassed Baton Pass team. I have been on the giving and receiving end often when playing Baton Pass, and I can appreciate both sides of the argument, and I therefore think that I can weigh in on the subject.
Baton Pass CAN be hard countered, but a lot of the time this requires:
suboptimal movesets and the element of suprise, eg Haze Gengar/Haze Dragonite. and competent Baton Pass players can play around and watch out for these kinds of CounterTeaming tactics, e.g. playing safe and kililng potential Hazers and then continuing the chain, instead of boosting in front of them, or Passing into Mr Mime on a suspected Perish Song.
opposing setup sweepers that can boost and overwhelm the chain, eg Mega Pinsir. Thing is, after you lose to them once or twice, you learn not to lead with Scolipede against a team with Pinsir/Talonflame in Team Preview, and can tailor certain slots to deal with this - I mentioned Zapdos earlier on.
Perish Song - Mr Mime is crap in OU but on Baton Pass it allows the chain to laugh at Perish Song users. however, you can leave Mr Mime at home and still win a lot of the time due to the lack of Perish Song on most OU teams.
Roar/Whirlwind/Dragon Tail/Circle Throw: users of these moves can be identified easily, and the use of these moves can be seen a mile off - pass to Espeon, have a Substitute up, problem solved.
Trick+choice item: Baton Pass chains should keep Substitute up at all times to avoid critical hits anyway, so this isn't really too much of an issue. Infiltrator+Switcheroo/Trick users are obvious as anything, just work around them, Pass into something to take the item, then Pass again.
A final overview of the state of Baton Pass this generation:
buffs: Scolipede, Sylveon, slower metagame
nerfs: roar/wwind goes through protect now, general power creep, sound moves going through Substitute, Infiltrator (to an extent), rise of Dark Types that scare Espeon (not really that much of an issue as i discussed earlier).
I think that, if something should be banned, it should be:
Pokemon that have the Speed Boost ability, when combined with other Baton Pass users
they should still be able to pass boosts on their own, just not as part of a full chain - and if you ban Scolipede people will just use Ninjask instead, trust me. are there any other Speed Boost Pokemon besides Blaziken that also learn Baton Pass?
if we were to make this complex ban, Baton Pass would be nerfed enough so that it would still be a legitimate playstyle, but not so nerfed that it is unviable, the Speed boost to everything is the biggest issue, as it allows the BP team to laugh at traditional speed tiers and stay a step ahead at all times. This would mean that us Baton Pass users would have to use things like AgilityPass Zapdos or Gliscor, which I believe is a fair tradeoff.
banning the entire strategy is a very bad idea in my opinion, as it leads to a slippery slope. Should we ban Kyurem-Black for its ability to pretty much singlehandly destroy Stall teams? or Gothitelle for its ability to cripple and destroy Stall teams on its own? or Mega Medicham for being able to dismantle large parts of stall teams? maybe Mega Venusaur should be banned because it arguably is centralizing and can wall and win against about 80% of the tier one on one (i've seen people running things like Return on Belly Drum Azumarill just to beat Venusaur, not to mention slashed moves in loads of analyses just so that something can get past Mega Venusaur)?
I have always been, and will always be, a dedicated Smogon player, and I believe that Smogon's tiers, rules and clauses are the best when it comes to balancing competitive Pokemon. I just think that, whilst banning Baton Pass wouldn't necessarily be the worst thing in the world, it could lead to a slippery slope - remember the arguments caused by "why isn't Blaze Blaziken allowed in OU, why isn't Mold Breaker Excadrill allowed in OU", which then lead to a huge stupid argument of "everything should be allowed in every tier with limitations" - I can imagine soemthing along the lines of "why can't we use 4 BP members in team instead of 2/3/whatever members" if we were to ban Baton Pass in certain ways, eg restriction of number of teammembers that can carry Baton Pass.
p.s. thank your lucky stars that Moody is banned in every tier, I played a match against a guy on Wi-Fi where he had Moody Smeargle with Stored Power and Baton Pass....