Terrain: Electric Terrain: Increases the power of Electric-type moves used by Pokémon on the ground by 50%. Prevents Pokémon on the ground from falling asleep and being afflicted by Yawn.
Grassy Terrain: Increases the power of Grass-type moves used by Pokémon on the ground by 50%. Restores 1/16 HP to all Pokémon on the ground each turn. Halves the damage taken from Bulldoze, Earthquake, and Magnitude.
Misty Terrain: Halves the damage taken by Pokémon on the ground from Dragon-type moves. Prevents Pokémon on the ground from being afflicted with status conditions; in Generation VII, also protects from confusion.
Psychic Terrain: Increases the power of Psychic-type moves used by Pokémon on the ground by 50%. Prevents Pokémon on the ground from being hit by moves with increased priority.
You see different terrain have different effect. There are no best, but in my opinion, Grassy is the best. Horn Leech+Leech seed+Leftovers+grassy terrain rocks.
Movepool: Different Tapu have different movepool. Fini has access Aqua Leftovers core, Tapu Bulu has Horn leech seed core, Tapu Koko and Lele is a strong attacker. If you want a attacker, Koko would ne the best, best supporter would be Lele, Fini and Bulu is a good balanced Pokemon. They just heal themselves while damaging the foe.
Typing: As for typing, I'd say Tapu Koko. It has different varaties of STAB move, and can deal with many Pokemon.
Stats: If you're wanting a sweeper, then use Koko. It has good speed and SpA. If you want a tank, then Fini. It's very bulky. Bulu and Lele are balanced.
The best Tapu I think is Koko.