Shadow Tangrowth

Shadow Tangrowth is a solid Grass attacker. While it falls behind Kartana, its overall performance is similar to that of Shadow Venusaur and Shaymin-Sky, which is definitely nothing to scoff at. Worth considering to grab a few to fill out the remaining spots in your Grass lineup.

Best PvE 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.