Tangrowth

Tangrowth is an okay Grass Type attacker.. and not much more can be said about it. Use it if you really need Grass Types and don't have better options.

Best PvE Offensive Moveset

• Vine Whip gets STAB and is the superior choice.
• Infestation is not only suboptimal in performance, but doesn't share the same typing as Tangrowth's role. 
• Power Whip is Tangrowth's best charge move with STAB, and is more efficient as a 2-bar move.
• Solar Beam performs a bit worse on paper, but can be inconsistent in practice due to how slow it is. 
• Sludge Bomb doesn't have much use on Tangrowth.
• Ancient Power isn't meant for Tangrowth in PvE. 

Best PvE Defensive Moveset

• Infestation is generally better on defense due to its higher base power and coverage typing.
• Vine Whip isn't that far behind since it has STAB. 
• Rock Slide is the most valuable on defense for coverage, and for being a 2-bar move. Ancient Power is a 3-bar alternative, but it is far easier to dodge.
• Sludge Bomb is a good defensive charge move overall due to its early damage window. 
• Power Whip is still ok on defense as it gets STAB, but doesn't cover against attackers with typing advantages. 
• Solar Beam is way too slow for defense. 

Vine Whip is superior to Infestation stat-wise, and it gets STAB. 

Power Whip is a strong Grass-type move that costs a modest amount of energy to use. Rock Slide costs less energy, and provides very valuable coverage against four of Tangrowth's weaknesses. Sludge Bomb isn't as useful, but can nail Fairy-types and opposing Grass Pokemon. The nerf to Ancient Power following the April 2020 rebalance makes it no longer viable due to Rock Slide offering superior damage output.