Which one do you consider your favorite, the most fun and/or useful?

Schooling and Power Construct are probably my favorite from a visual standpoint. Transformation during battle is something I like and we see a lot of it this generation.
Other than that, I actually find Golisopod's Gimmick to be really interesting, both in First Impression and Emergency Exit. I wish it wasn't so harmful to it though, even just being able to choose to switch out or making it activate after you attack would improve it a lot.
Battery and Stakeout are really fun abilities too, I hope more Pokémon can get them in the future.