It's currently Volcarona, merely for the breeding I've been doing. I'm also raising it to 100 so I can get Feiry Dance. Normally I carry Feraligatr and/or Blastoise with me :3
I can't think of any pokemon that I ALWAYS had on my team, but the one I've abused of the most over the years is Charden, the Charizard I'm with since my first Fire-Red playthrough. Always transferred him over to the newer gens, and often used him as my flier and to spam around flamethrowers with.....Yup, Charden
i havent been playing white as much only about 2 days orso, bot 5 badges. but Serperior i ALWAYS have with me. if my stupid sister didnt lose her Nds could trade some awesome starters like cyndaquil to pokemon white. but i will have to wait for a while
I'll usually keep a tyranitar on my team, and often times have a salamence (favorite dragon) as well. Every option I can I have a typhlosion as well.
As soon as I have it, a shiny umbreon too.
There's no one pokemon I keep with me, but I'd have to say Chatot for RNGing and fly. I'm also currently raising a shiny Axew line, which will probably stay with me. I love the shiny Haxorus sprite.
To get the obvious out of the way I can't have a team without my starter it's like having an army without the general. Other than the starter I always have a place on my team for that unique yet slightly overpowered pokemon like these guys: ;461; ;448; ;292;
Well Rayquazza ;384; has been with me since Ruby, but after he reached Lvl 100, I stopped using him because Lvl 100's are too overpowered for me -.- but in every game since platinum, I use a Weavile! ;461;
Well I always have my Feraligatr from HG with me. Would've taken the one I had in Gold but I can't :| I also usually carry my Mewtwo with me, but that's so I can just destroy anything the game throws at me if I'm not otherwise ready for it. And I usually try to keep my starter from that game with me :3