All of them quite frankly.
Koko and Lele will initially be fucking everywhere, Koko just has too amazing a typing and synergy with a bunch of stuff. Lele being able to block all priority is just far too huge an implication.
Bulu and Mini will be slower to pick up steam but once people figure them out will also be really common. Misty Terrain cancels out all status so it forces everything into a brute force fights and trading blows.
Bulu has the biggest implications for a more technical player, Grassy Terrain has a decent amount of team setups you can run. It culls one of the most powerful spread moves and has some potentially more powerful support options.