I'm talking about pokemon that are immediately defined as gimmick, regardless of the set - I.E. Pelipper, Plusle/Minun and to a lesser extent Ninjask and Medicham.

Fire away :)

I'd say Corsola :)

Shedinja and Clefable.

Trapinch, Marowak and Clamperl.

Clefable isn't gimmicky IMO when using standard sets, especially in UU. Although, it does carry a lot of potential for gimmicky sets, but, I don't think the Pokemon itself is really gimmicky.

Spinda is a master of gimmicks with its craptastic base stats but great movepool. When people try setting up (via Dragon Dance or something) against Spinda (probably not knowing what it's going to do) and you use Encore or Trick, haha, awesome. Its sprite is a gimmick too...

Luvdisc!

Castform xD

Male Combee, Shuckle (probably), Kakuna, Pyroak

Most NFE Pokémon. I have a team made entirely out of them :P

I'd say Wobbuffet (and Wynaut) with a whopping 8 moves yet with the coolest ability in the game and an Uber.