Well, in my current playthrough of White, I have a few Pokemon that seem to be in my party a lot. A bunch of them are HM slaves; others are useful for catching Pokemon, or I just like them.
Stoutland: I think she's the Pokemon who's in my party the most. I've had her in my team a lot lately when exploring a new area, since she knows both Dig and Strength. She's pretty strong too, so is useful for battling trainers as well.
Swanna: Surf, Fly, Roost, and Featherdance. I wouldn't have any other moveset on a Swanna. Makes him both a useful HM slave and a decent battler.
Zebstrika: I used to have her in my team a lot whenever I explored a new area, because I like to have her use Thunder Wave to paralyze a Pokemon I'm trying to catch. But now that I have a Galvantula with Thunder Wave, I might just have her fulfill this role instead.
Audino: He's an awesome support Pokemon, so I always bring him along if I anticipate lots of double battles, and frequently for training in the double battle grass as well. He also has higher-than-usual Attack, making him able to stand his own in a single battle as well. His current moveset is Secret Power, Heal Pulse, Helping Hand, and Attract. I love it.