My top three favourite picks for Mario Kart Wii are Daisy in Royal Rayer, Wario in Offroader and Diddy Kong on Bon Bon.
In Super Circuit, I played Peach in the past, and mained Toad later. I'm now trying to go towards Bowser, because you get more points for him, but as he was a bit too hard to control for me, I now play as Donkey Kong. Wario controls too badly compared to the amount of points you get for using him, in my opinion.
I've only recently started playing Double Dash, and in there, when items don't matter (time trials, battles) I like Wario and Daisy, in Daisy's car. In GP's I don't really use Wario, because there are much better items than the bob-omb.