What is your favorite Mario spin off series?

For me, it's defiantly the Paper Mario series. I find them even more enjoyable than the actual series. The plot of the third one is great, but really dark for a Mario game.

Spoiler:

I also like the Paper Mario series as well. And I also like the Mario Kart series even though the later games seem to be not as good as the previous ones, imo. :/