I've finished one playthrough and started another one to test some more mons. I'll comment on the ones where I feel a statement is most important:
Slaking
The Slakoth period was bad. For some time, I did switch-grinding, then I turned on Exp Share for a little while because I was getting tired of it, only switching it in for battles that give a lot of exp like the double ones. There is no doubt that Slaking needs to be penalised for this period and its length.
As Vigoroth, this guy is quite comparable to starters' second tier forms in power except that he doesn't have a super-effective STAB. I ran Strength/Power-up Punch/Bulldoze/Shadow Claw (since I also used Mawile on the same run) on mine, and most of the time it was OHKOing things with SS-boosted Strength. Bulk Up is a choice but is obsolete when you can just get the same power boost with PuP while striking a weakness. Sometimes I gave it Rock Tomb and/or Low Sweep depending on the area I was in.
Though Vigoroth was more than good for the period he existed in, I believe he doesn't need separate tiering because Slaking is simply superior in spite of all the Truant limitations.
Now, about Slaking. You can buy Giga Impact TM about as soon as he evolves and for a good while it's kinda overkill because Strength should be OHKOing most things. Hammer Arm can be tutored but it's dubious that you'll want to hit something like Golem or Rhyperior with it. Really, you'll be mostly using normal-type moves with Slaking, opting for something like Earthquake for that perfect accuracy when you know you're grabbing the kill anyway.
I believe Colonel M was estimating how many turns it would take Slaking to sweep. I don't see the point of these calculations because Slaking is never going to sweep anything. However, the inability to nuke a lot of targets, potentially very problematic targets, is invaluable. For example, you lack somebody who can reliably and quickly KO Wallace's Milotic so you just Giga Impact it - one-shotted as you would expect. It put a huge dent into Primal Groudon too (~70% HP gone) and I kept OHKOing tougher mons in the E4 even at a 10-15 level disadvantage.
By switching Slaking in, you can continue making great contributions to your team and Giga Impact PP is not a problem because Leppa Berries exist. Slaking offers S-tier Mega form level offence (possibly more) while contributing to the team in small bursts - these bursts can mean more than half of the team OHKO'd (odd number of Pokemon in a trainer's team) to somebody's entire team (one-mon teams are often the hardest generic trainer battles in the game).
At lower levels, Slaking has other utility - taking hits when the team is in trouble and outspeeding many targets due to high base speed. It is a good idea to leave Slaking with Counter or just tutor it to him before the E4 - I had trouble with Steven's Mega Metagross for example and Slaking is just healthy enough to take a Giga Impact / Meteor Mash from him and KO him with Counter. That moveslot can't be considered wasted because Slaking pretty much needs just one move when you get it.
Truant can definitely be worked around with such stats, so I'd like to nominate
Slakoth for A/B tier and discourage Vigoroth from being tiered separately.
Next one I'd like to talk about is
Mawile.
Great bulk and typing no doubt, and I must say I was mistaken when I said it's bad without the mega evolution. If your team is struggling at a hefty level disadvantage, it can be a good idea to mega-evolve somebody else (you have like 20 other options in this game lmao) and keep using normal Mawile as an Intimidate switch-in that can still tank certain mons well and can still deal respectable damage with STABs and Stone Edge (referring mainly to the E4 period where Sceptile was often the one who mega evolved instead of Mawile).
It's doing a good enough job pre-Fallarbor when the tutored STABs start seriously breaking the game even before you get the Mega evolution. Limited utility against Wattson however, as you don't have Low Sweep or Bulldoze access, and pointless against Flannery. In the upcoming battles however, it is very hard to not find some usage for Mawile and he can pull even a very weak underlevelled team ahead just by virtue of being there when you need him.
Mega Mawile has overkill power, moreso than Slaking (but his moves have lower BP so the two hit approximately equally hard for me) but nothing is overkill when you're facing somebody 15 levels above - I failed to OHKO Cradily with Iron Head as a mega for example. Setup seems largely unnecessary but I used PuP quite a bit, for instance to get back the attack lost from enemies' Intimidate switch-ins. Swords Dance + Sucker Punch is a good enough strat against Phoebe, too, but generally low speed prevents him from being a good setup sweeper. You don't mind though.
I'd say Mawile needed the least healing on my playthrough because of his typing and bulk (Slaking also didn't need much for obvious reasons, and Sceptile needed a good bit because Mega Drain was the hardest draining move on it). It was easily the MVP of the run.
Mawile's only con is having to backtrack to Granite Cave but the time required is rather acceptable, I've found. You can instantly tell which Mawile has Intimidate upon their appearance, so you spend less time dedicating to its capture, and you don't need extra Pokemon to catch Mawile (kinda like you need another Surfer to get Carvanha, or Oddish to get Magnemite). I definitely spent a lot more time trying to get a Corphish on the same run, and I spent about the same amount of time getting a Magnemite on my current run even though I didn't have to backtrack. It's a shortcoming to be remembered but not sufficient to warrant a tier drop in itself.
Mawile is quite likely Omega Ruby's equivalent to what Honedge was in X/Y. I'm nominating him for S/A-tier. While backtracking doesn't take so long, let's not forget Mawile is quite slow as well (not really noticeable early on) and won't take neutral special attacks when out of mega form.
I'd argue that, all things considered, he's a better steel-type than Magnemite (another serious candidate for S-tier).
Quick tierings for other teammates:
Treecko - A tier (below the big five, but above Carvanha, Lotad, Geodude and others currently present in A who still need to drop - the tier really needs to be split IMHO)
Numel - B/C tier (hard-hitter but slow and not all that bulky, afraid of water even with Solid Rock)
Corphish - B tier (late availability; mine had a +spd nature so it outsped more than I'd expect; all things considered, heavy hitter with good physical bulk and diverse movepool)
I don't see the discussion on Treecko ending anytime soon, especially since its the only non-S Rank of the ORAS starters. And since the pledges issue varies from player to player, it also means that their experience with Treecko would be different from player to player. Someone with a team of two or three going into Mauville would fare very differently with Treecko than someone with a bigger team by that point in the game.
Of course, the discussion on Treecko's tier might betray a very different discussion. The XY IGT List has 4 S-rank mons: Aerocadtyl, Charmander, Honedge, and Riolu. By contrast, ORAS's current IGT List has 9 S-rank mons: Alakazam, Carvanha, Lati@s, (counting them as one because they're very similar and you get a different one in each version, so its more like Megazard X vs Y than 2 separate pokes) Lotad, Magnemite, Marill, Gardevoir, Mudkip, and Torchic. The ORAS tier list's S rank is much more cluttered and I think that's why a lot of people want are upset that Treecko isn't there. Just a hunch, and I'm not trying to denigrate any of those 9 mons of course.
It seems clear that Treecko wants his Pledge the least out of the three starters. Marshtomp wants it the most because Water Gun remains the best water-type STAB until Surf otherwise, and Combusken wouldn't mind a move stronger than Flame Charge considering he's equally good at special attacks. Meanwhile, Treecko can get a very early and potent Giga Drain that has almost the same power but far superior utility to the Pledges.
If the difficulty of access for Pledges is a problem for anyone, it's Marshtomp first and foremost, but Grovyle the least.
I think the only thing that kind of hurts Grovyle and Giga Drain is, ironically, its Speed. It's almost always going first so you have to hit with something weaker or attempt to Giga Drain the next Pokemon to really make full use of it (unless you're injured from previous fights).
One thing I'm really liking though is that somewhat early Swords Dance with Grovyle will prove useful later on. Swords Dance / Dual Chop / Leaf Blade / fillers will prove undoubtedly useful. Yeah setup sucks but Megatile should be able to pubstomp any trainer (and I'm sure Sceptile can pubstomp normal trainers - especially if I went and taught it Leaf Storm instead).
I think some of the arguments against Treecko are a little strange, this one included. If he fails to KO, he will be hit - the damage could be big depending on the attack. Next time you attack, you drain the HP dealt back, and choosing when to use the mon can imply you don't need any healing at all. That was the case while I still had Mega Drain and it wasn't obsolete, and the same would have been the case if I postponed the evolution a little bit.
And assuming Treecko gets hit never (quite an exaggeration really) would be a good thing and not a bad one, right?
I also heard that Overgrow somehow "spoils" the recovery from Giga Drain - what's up with that? If you're in critical HP, you'll just drain more HP than you otherwise would. Sceptile is also the best user of his primary ability as a starter (assuming non-mega) considering how much faster he is than the other two. What's the use in Torrent or Blaze when you're risking being KO'd by not being fast enough?
Sceptile's speed is also a big boon, especially in the last portion of the game where grabbing kills can get quite hard and getting in that last chip to take a tough foe down without getting hit is pretty important.
Leaf Storm is also a very big move in-game and makes Sceptile a real hard-hitter.
On the other hand, Sceptile isn't bulky enough to ensure a reliable sweep in the important battles. However, sweeping isn't the sole requirement to be in the highest tiers. What is Swampert sweeping for example? Not a whole lot even with Rain Dance and PuP.
On the topic of DexNav, are there any definite statistics available on the probability rates of certain abilities, breeding moves and higher-than-normal levels appearing? For example I have a Breloom with Drain Punch and this thing really packs a wallop but how likely is a player to find the same Shroomish that I did?