Pokemon is the only series where I prefer the spin offs but that's only because of Colosseum and XD.
Other games? Not at all, because they're usually mediocre at best. Wario Land is the best example of that. (Just to clarify this has nothing to do with Warioware, which I like.)
I guess I DO like Uncharted: Golden Abyss. And Tekken Tag Tournament 2.