Generally I agree with what AM said about Thundy-T; it isn't quite as outclassed as the days of BW/XY/ORAS due to the Prankster nerf, T wave nerf, emergence of Psychic Terrain, and speed creep.
However, Thundy-I's speed gives it a big advantage over Thundy-T, as it outruns offensive Garchomp, Mega Metagross, Mega Pinsir, Keldeo, Nihilego, and whatever else is between 101 and 111 (especially the crowded 110 speed tier).
I'd say Thundy-T's NP sets/double dance sets give it a decent niche, possibly enough to warrant a rise from C to C+, but I am more in favor of Thundy-I dropping than Thundy-T rising. B+ seems okay but it feels more comfortable with B-rank mons due to its splashability and consistency.
Here's a nomination:
Tangrowth to A-.
The main reason I believe Tangrowth is worth a rise from B+ to A- is a matter of how it's been shown to consistently check a lot of top threats with its sets. AV is excellent for handling electrics, Greninja (ash and protean sets alike), Manaphy (a big perk for fat teams), Zygarde, Magearna, rain attackers, and all sorts of other mons that can be hard to deal with. It is fairly splashable and can fit onto everything from fat balance to bulky offense. As for Physdef, it checks Mega Metagross well, and the meta is evidently in serious need for an answer to this S-tier monster. Additionally, Terrakion, Bisharp, Jirachi, Buzzole, Tyranitar, etc are either annoyed or beaten down. Both sets take on Defensive Lando-T, which is everywhere atm, Rotom-W, Dugtrio, Tapu Fini, Ferrothorn, Tapu Bulu, Garchomp... Need I continue?
Now, everyone here knows what Tang accomplishes; I merely want to highlight that there a high amount of OU mons that Tangrowth beats, especially those that are seeing very high usage on the ladder and in tournaments. It offers a good blanket check and role compression in a meta where prepping for every threat is nearly impossible, and that it Tangrowth's greatest asset currently.
It isn't outclassed by Amoonguss because A: It resists ground, B: It can be customized to be a better fit on teams (if the team needs Physdef or AV then Tang has options, whereas Amoonguss does not), and C:
It 1v1s Meta Metagross, Garchomp, and Zygarde.
Final considerations:
SPL Week 1: 13 | Tangrowth | 7 | 11.67% | 42.86% |
SPL Week 2: 10 | Tangrowth | 5 | 16.67% | 40.00% |
It's seeing fair usage in SPL, more than a lot of great mons (Hoopa-U, Skarm, Zard X and Y, Volcarona) in fact, and with a fairly average win rate to boot.
http://replay.pokemonshowdown.com/smogtours-gen7pokebankou-234706 Tang checks Rotom-W consistently and supports Mega Meta in doing so.
http://replay.pokemonshowdown.com/smogtours-gen7pokebankou-234776 Tang cripples Mega Meta with sleep, PDC's own Mega Meta KOs it and gets an attack raise while doing so to clean up the game.
http://replay.pokemonshowdown.com/smogtours-gen7pokebankou-237196 Tangs put in work on both sides, consistently checking the other's team.
http://replay.pokemonshowdown.com/smogtours-gen7pokebankou-236898 Despite losing to sun offense, Tang nets a surprise KO on Tapu Fini.
http://replay.pokemonshowdown.com/smogtours-gen7pokebankou-236798 Tangs are critical to both players and blanket check each team.
Tangrowth puts in work frequently and is very effective against the mons that are trending currently; as such it merits a rise to A-.