Sorry to do this just as you are considering other nominations, Omfuga, but as
Christo The Gr8 and I agreed in the UU PS Room chat, we'd like to bring up
Mega Beedrill to S. (Also, apologies for rambling a little bit, I tend to think as I type so I end up writing more than may be necessary, probably why when I used to write for a living I had an editor lol)
I'll start by saying that some time ago I actually thought Mega Bee should drop. I've built a lot of teams, played a lot of games with and against Mega Beedrill and reconsidered my opinion since that time, quite drastically. As I've said in the same chat, 99% of my teams I build these days have Mega Beedrill (or Mega Abomasnow) on them. I think the reason is obvious.
MBeedrill's incredibly high attack, speed tier, and offensive typing all accomplish exactly what they intend to in this meta. The argument for Beedrill actually reminds me a lot of the discussion we all had regarding Entei some months ago when he was nominated for S rank as well - though it can be predictable, if a mon accomplishes exactly what they are put on a team to do almost 100% of the time, can they be anything less than S?
It's a little harder to put into exact words than Entei was, though. Beedrill's strength, I think, lies primarily in the fact that it synergizes so well with so many other great mons in the tier. It has very few outright counters among the viable mons of UU. Take for example the rest of the S rank:
Mega Aero: one of the main reasons MAero remains this high in the viability rankings is the fact that it outspeeds MBee. While MAero has a ton of other positive attributes, if that isn't a flat out sign of viability I don't know what is.
Feraligatr: Can come in on rocks one time before it is OHKOd by Mega Beedrill. Needs to be running Aqua Jet to pose any sort of threat, as it doesn't even outspeed Jolly Bee variants at +1 with the Dragon Dance set.
Hydreigon: Needs to be wearing a scarf to not have to switch out. If Hydrei is not scarfed (which is, I think commonly agreed), the lesser of its popular sets at the moment, then the Hydreigon user immediately either forfeits a ton of momentum or sacks a mon to MBee.
Salamence: Will be U-Turned out on, often comes in on rocks (as it is used commonly as a defogger), taking at minimum 45% just to switch in to get the intimidate off and usually be forced out by another mon.
So I guess what I'm trying to get across is that, due to it's STAB Adaptability U-Turn, even "counters" to Mega Beedrill that can take a hit and OHKO back usually don't function that way. I understand that the mons listed above are primarily offensive, but Beedrill fairs just as well against defensive mons, either outright slaughtering them (Cresselia, Reuniclus, Florges, Slowking) or denting them significantly and forcing them to Rest/Recover in some other way (Snorlax, Suicune). The problem with switching into Mega Beedrill, as I'm sure many of you know by now, is that you are immediately under pressure again the turn after because you're usually switching into a U-Turn. For a more practical example of what I mean exactly, please watch this UUPL match featured in the Better Battlers project:
http://replay.pokemonshowdown.com/smogtours-uu-58019
In this high level match, Beedrill facilitates nearly everything dodmen accomplishes. It set him up to get easy rocks within the first few turns. It keep Alomomola stuck in a Wish/Protect loop. It displays synergy with just one other VoltTurn mon, the S Rank Hydreigon (so, not exactly uncommon), to bring about a Slurpuff sweep. Hazard stacking plays it part and proves once again how effective a strategy it is when combined with a mon that doesn't hit and run as part of a defensive, long term strategy, but as an active and outright offensive one. Simply put, there are almost 0 chances to Jirachee to respond to Mega Beedrill in this game, and it keeps momentum firmly on dodmen's side.
I guess the last point I'd like to make is actually in favor of the mandatory Protect in Bee's arsenal. Folks are probably used to arguing that this was a negative, but Protect is actually a great move on this mon. Considering that, aside from Mega Aero, you're typically going to need a Scarfed Mon to revenge Beedrill, Protect again puts the momentum in the favor of the user by easily scouting what move they're locked into. Christo proves this in the following replay, allowing him to easily work around a scarfed Mence and keep control of the game:
http://replay.pokemonshowdown.com/uu-255950592
In conclusion, amazing offensive typing, ability to aggressively pivot out without sacrificing momentum (think like a Lando-T might in OU, where the U-Turn won't hurt very much), top-tier sweeper stats, synergy with other VoltTurn mons, synergy with hazard-stacking cores and ability to facilitate sweeps for other setup mons (Gatr, Slurpuff) mean that this is one of the top premier threats of our metagame at the moment. I think anything Mega Beedrill definitely deserves S rank.