Well, Signature Pokemon are kind of tough to use, as each game has different Pokemon in them. Most of the Pokemon I like to use you can't get in the main game, at least until after you defeat the Pokemon League and recieve the National Dex.
Sableye is insanely useful, especially if you grab one from out of the Dream World. I really only use it to make myself happy, not really practical. Scrafty is also a staple ever since Black came out. Dragon Dance Scrafty is unstoppable. I bred it to take on the Pokemon League and at level 15 it was taking down level 40s in just a few turns. Honestly if I was given a game and they said "You may only use one Pokemon in this playthrough and it can be any Pokemon you want) I'd pick Dragon Dance Scrafty.
Really it depends on the game I'm playing which one I always play with. I'll list them Game by game.
Red/Blue/Yellow - Mankey/Primeape
Gold/Silver/Crystal - Totodile or Cyndaquil, Ampharos (G/S), Arcanine (Crystal)
Ruby/Sapphire/Emerald - Mudkip, Mightyena
Diamond/Pearl - Luxray
Black/White - Scrafty
I haven't played BW2, so I imagine I'd still primaritly use Scrafty.