Still Celesteela, for me. Its stat distribution and moveset mean it can run whatever set it wants, whether it's the traditional defensive set with Leech Seed, or a more offensive set with Autotomize. Quite a lot of the other Ultra Beasts in comparison are pretty one-sided in how they perform - Kartana for example is a pure physical attacker. I've also liked playing with Pheromosa as well, with the high speed and offensive stats, it can also have a surprise factor.