In Melee, I usually play as Peach. I also like to use her in Mario Kart. Folks don't take me seriously whenever I play as her which allows me to do more damage to them.
I'm not sure when or if I will get Brawl. I may get it for the summer when I have most of my parties at my place, although I want Mario Kart and will probably buy that first. I'm sure that I'll stick with Peach though (in both), especially if her moves are similar to what she does in Melee.