Tangrowth @ Rocky Helmet
Ability: Regenerator
EVs: 248 HP / 204 Def / 56 SpD
Relaxed Nature
- Giga Drain/Leaf Storm
- Sleep Powder
- Knock Off
- Hidden Power [Fire]/Hidden Power [Ice]/Stun Spore/Earthquake/Leech Seed
Physical EVs let you dodge things like CB Azumarill Play Rough 2HKO, while the SpDef investment lets you take a LO Starmie Ice Beam and not die (or 2 Specs Keldeo Scalds and not die). Relaxed is best nature since you're slow and you don't want to compromise your great mixed offensive stats or your defenses.
Giga Drain versus Leaf Storm is your preference, as Giga Drain gives you valuable recovery (you really don't have a moveslot to waste on Synthesis), while Leaf Storm lets you stop Manaphy from setting up on you and does a ton of damage to things like Mega Gyarados.
Sleep Powder and Knock Off are mandatory for utility: the former lets you sleep incoming counters like both Zards, Thundurus, and Talonflame, while Knock Off is great and you often get to use it as they switch out their slept mon for a free item removal.
Last moveslot depends on what your team needs. Tangrowth has great mixed offenses for a defensive grass type (100/110 is damn nice), but you can even run Stun Spore for that double status that nails ground types (very nice versus Lando and Chomp). HP Fire is aight since you can force out Scizor and Ferro, but IMO HP Ice is far superior, since you then counter Landorus-T and physical Garchomp with ease. Earthquake lets you hit Excadrill/Metagross/Heatran which is good (especially for the last one on the switch, a clean 2HKO), while Leech Seed is for general recovery.