Because the game has been datamined, we have enemy stats here:
http://pastebin.com/vkaZNTra
I wanted to use these to get a snapshot of Pokemon offense, so I used a damage calculator to plug in a pretty neutral enemy Pokemon (Gladion's Type Null at Level 38 which has 0 EVs, 95 base stats in everything, and 30 IVs in everything) and see how much damage the Pokemon from tiers B - S (plus some scattered mons from the other tiers) would do against it. I took every Pokemon at Level 40, assumed 15 IVs, neutral nature and 20 EVs in every stat and calculated damage output with each Pokemon's "best" Level 40 move (though I cheated in a few spots like with Bruxish and Psychic Fangs since it learns it at 41). I also plugged in natures/IVs for trade Pokemon like Bouncee (Adamant nature, 31 Atk IV). The result was this list:
http://pastebin.com/mDZjxpfz
[Note that Ghost moves were changed to be able to hit Type:Null and fighting lost SE so I could remove unfair advantages/disadvantages to Ghost/Fighting Pokemon]
This list isn't meant to take the place of a tier list by any means because there's still a lot these numbers don't tell about the Pokemon. They don't tell us when they get the move they're using, how they fare against non-neutral matchups and much more. However, I like it as a decent starting point to show how a Pokemon's mid-lategame offense is. An example of how this list needs to be supported with context is Sylveon, who has one of the higher damage outputs, but doesn't get Moonblast until Level 37 and is a liability beforehand.
I learned a lot making this list and to start want to propose five changes to the list:
Diglett to B: 100 Attack and a 80 BP STAB seems good on paper, but it actually lags behind the majority of B-S Pokemon in mid-lategame who mostly either have a stronger attacking stat or higher BP STAB. With the Diglett stage already heavily criticized for starting weak, I'm not sure I see the payoff at the end when Dugtrio is struggling to OHKO even after an X-Attack. Its typing is excellent, but Earthquake comes too late at Level 47 and I don't really ever see where it has a dominant period which it needs to be A Tier caliber.
Comfey to E: I've already joked with Colonel_M about this one. How the hell did it get put in B? Even
with Petal Blizzard or STAB Dazzling Gleam (both are 120 BP moves effectively) it has one of the worst damage outputs of the B-S Pokemon, and it needs to wait until its 40s to get there!
Eevee (Espeon) to A: Its damage output with Psychic running off 130 Base is drool worthy and its very fast (especially for Aloha). With the Psychic TM coming after the first visit to the Aether foundation, it should easily sweep through anything that doesn't resist its type. Kadabra/Alakazam's performance in the earlier games comes to mind.
Bruxish to C: I understand that its a pain to catch a Level 30 one, but its damage output is top notch. Psychic Fangs isn't until Level 41, but even non-STAB Crunch was placing its damage output alongside Hydration Pelipper/Machamp when I ran the numbers. D is way too harsh for it
Fletchling to B: Its damage output with Acrobatics is basically equal to Drifblim's, except Driblim has better secondary coverage (Talonflame's fire movepool is almost non-existant before the Elite 4), and is much much tankier. Either Drifblim is better than we think, or (more likely) Fletchling isn't up to par.
Please don't hesitate to point out any errors in my pastebin, and I might consider doing this at another point in the game (likely earlier in the game) to see another snapshot of how everyone's offense grades out. I could probably go on with about twenty more changes based on doing that exercise (A tier Drought Torkoal and Toucannon anyone?) but I figured five is good to start with.