Charjabug, simply because it's my most favorite Pokémon. I used an Eviolite on it though, so that might be cheating.
During my B2W2 postgame playthrough, I realized just how bad Castform, Lunatone, and Tropius were as well (as much as I love them). Castform's stats were awful and its one gimmick wasn't the safest to play with, Lunatone's typing and movepool made me wonder what I was supposed to do with the dang thing at one point (Rock Polish, Calm Mind, Shadow Ball, and Earth Power did the job, at least), and Tropius's speed and Solar Power made it a bit of a liability.
I tried making Seviper work in Scarlet's Victory Road storyline as part of a mono-Poison run... It sorta pulled its weight, and even landed the finishing blow in the final battle of the story, but during the Elite Four it was mostly a liability if it couldn't set up a Coil or two.