SMK: I main Peach, but I dislike that Bowser is her rival because of his OP Fireball item that is tricky to avoid. I could always circumvent this by plugging in a second controller and having Bowser be P2... or use Yoshi. I'm a noob at this game, so I need these high acceleration characters.
MK64: Peach for Grand Prix and versus. I will say that it's nice to have her voiced by Leslie Swan in this game.
Luigi for Battle mode because I won't let Peach get hurt.
MKSC: Peach for Grand Prix and versus. I managed to three star all of the super Circuit cups on every difficulty with her.
Again, Luigi for Battle mode
MKDD: I'd say that Peach and Birdo are my favorite characters to pair up in this game. But I only use them when I play for fun. When playing the higher cc's, the lightweight karts are just infinitely more useful because of their long-lasting mini Turbo making them the best for snaking. I like Toad and Toadette team and Koopa + Paratroopa
MKDS: Peach is my fave obviously, and it's good that you can use any kart with any character in this game which means I can use Dry Bones' overpowered Dry Bomber or the Egg 1 with Peach.
MKW: Baby Peach, Birdo, and Rosalina. That's one character for each weight class.
MK7: I like Rosa here the most probably since it's the only time she's voiced by Kerri Kane. Also like Lakitu.
MK8DX: I use like half of the cast, but will say that Peachette is my #1 favorite. Even the stats page say so since I've played as her the most.
MKT: Love the Peach alts, Birdo colors, Dixie Kong, Toad and Toadette alts, Pauline and her alts. And for online 1 or 2 item races, Baby Peach and Wendy.