This was actually harder to answer than I thought it'd be...my first thought was actually Binacle/Barbaracle, but that was my FIRST thought, and it's primarily because it's at the bottom run of badly designed Pokemon I don't like, so it's memorable in that respect. I actually had to think about this and browse through a list of Pokemon.
For me it's this:
Poor Druddigon. It's just kinda there. A mismatched Crayola Dragon, forgotten and overshadowed by every other Dragon, for good or ill. It's not a BAD design, like Turtonator or the Jangmo-o line, but it's far away from the likes of Gible and Flygon. It's not bad as far as Gen V goes, but it's not great either. It's average and ulitmately forgettable for it. I almost feel sorry for it.