Tangrowth is actually really quite good. When I said in passing before that Tangrowth was probably a B rank, I was being quite conservative. With an assault vest set, tangrowth is bulkier than mega venusaur, and with 4 attacks and nearly as good attacking stats, can maintain a comparable offensive presence. It is, in my opinion, the best like-for-like replacement for venusaur if you wish to use a different mega pokemon. The best set is probably something along these lines:
Tangrowth @ Assault Vest
Ability: Regenerator
Relaxed Nature
252 HP /4 Def/ 252 SpD
- Leaf Storm
- Knock Off
- Earthquake
- Hidden Power Ice
IMO leaf storm is actually pretty critical. Without any attacking investment, you need it to OHKO things like rotom-w and manaphy after SR, and frankly the special attack drops are hardly an issue. You switch all the time as it recovers you, and you have two respectable physical moves to fall back on. We all know how annoying knock off is, and it does pretty good damage to the likes of Lati@s, while EQ mauls heatran and HP Ice deals with Garchomp, Dragonite, Gliscor and Landorus (-T).
What the calcs the above user gave show really is not pokemon that tangrowth counters, but that Tangrowth is a hard pokemon to set up on, and that if you can get some kind of hit off on a sweeper, tangrowth can usually tank a hit and finish it off. Having sweepers set up on you is what you would normally worry about for a wall like tangrowth, particularly one that is incapable of crippling with status moves because it carries an assault vest.
The pokemon tangrowth actually does counter are many. Just looking at the beginning of this thread, you counter sweeper deoxys-S, garchomp (non-mega, most of the time), manaphy, landorus-I, Gengar, Keldeo, CB Azumarill, mega gyarados and more. You'll come out on top one-on-one against a huge majority, including aegislash, special mega lucario, heatran if it doesnt burn you, latios, thundurus, excadrill. This thing just does not die.
In conclusion, I think Tangrowth deserves to be in B rank because it is somewhat eclipsed by mega Venusaur, but belongs in B+ because it is actually really quite good.