For me, it's a close race (har) between Star Wars: Episode I - Racer, and Sonic & All-Stars Racing Transformed.
Ep I Racer does an incredible job of translating the sense of speed and danger of podracing from that film into something compelling but not unmanageable, hurtling around various planets in vehicles that can and will fly to pieces in a turn taken wrong. Plus, it gets the nostalgia boost, which does a bit of lifting for it...
Sonic & All-Stars Racing Transformed on the other hand is just flat out one of the best kart-style racers ever made. The aesthetics are big, bright and bold; the handling just Feels right and has that perfect sweet spot of very low skill floor and relatively high skill ceiling; Sonic makes that weird WHARGARBL noise when you crash sometimes. It's pretty near perfect in a way that Team Sonic Racing and Crossworlds, um, aren't.