Probably a tie between Gardevoir and Gallade for first. Gardevoir mainly because it (not referring to as a she cause they can be male, which surprised me at first) became the centerpiece of my R/S/E teams. Gallade is just an awesome physical contender, and while not the most powerful one on the physical department, Close Combat / Night Slash / Psycho Cut / Ice Punch can offer some decent type coverage and if you can last a couple of hits for that 125 base Attack to kick in, you're going to kick some ass. And of course, you can see who's the star of my avatar, made by a friend.
Second would be Shedinja. Like someone said, Shedinja is a Pokemon version of Candlejack (read it's Pokedex? Stare at it's backside and you're gone forever, your soul donated to Shedinja). Yeah, that 1HP thing does suck, but it can make it worthwhile using in Ubers... At least until a major threat comes in and uses Sandstorm upon switching in (like Tyranitar) and kills it on the spot.
Third, fourth, fifth, ect... I haven't really decided to be honest. These change often, depending on my mood and battling preferences.