I'm almost always Bowser. Because when I'm playing for fun, I honestly don't care if I make Bowser look like an idiot. Plus, he's totally awesome and stupid. And I like villains. Playing as Bowser and messing around is so hilarious... especially because of the noises he makes. xD Auuguhgaa... Ouuuwwoohhg... Booo'oh. Amazing voice-acting right there. Sometimes it is so worth it to screw up just to hear those sounds. ;P
Toadette is normally my second choice, just because she looks cool. Bahaha... but she can be annoying as a CPU. Well, then again, I guess all CPUs can be annoying... ;/
Wario is always awesome in almost every appearance. I like Bowser but I hate how he is always the slow lumbering one and the slowness normally annoys me. Wario has that, bash everything in his sights no matter what gameplay which is just great. I like it because it gives me a few extra methods of attacking beyond just jumping like normal. Donkey Kong is similar but I just never really liked the character much.
I Actually hope they make more games built like Mario 64x4 or American SMB2 where you have a variety of various characters to play in a regular platform setting. I love the variety that gives you as it lets you chose the character best suited to your own playstyle instead of always conforming to the ordinary playstyle of the game itself.
I almost always pick Wario if he is a choice. I love Wario, I don't know why I just do.
At the moment I've been a little money hungry, so I've bonded more with Wario.
Other than that I would say Bowser.
Well, for Mariokart, it's either lakitu, koopa, or baby mario/peach. For mario party, it's toadette and koopa. Smash Bros? Pikachu, Toon link, and Wolf. Paper mario? (Partners) Goombella. And for Mario Strikers Charged, Peach or Diddy Kong with partners toad, dry bones, and hammer bros.