Moved this over to Pokémon General, since a question like this isn't always game-oriented and can focus on the franchise as a whole!
But anyways.. for me, Galvantula. It's a good mon in the games and hardly ever used or talked about within the franchise in general. Also Bellossom. Sure, not as good as Vileplume, but still super cute!!
Milotic, run max SDef Milotic with mirror coat and it can take any SDef threat. Agasint a volt/turn team with Rotom? Not anymore because Milotic eats that Volt Switch and nukes whatever comes in. 😀 This Milotic set has claimed many souls for me in UU.