Mario Karts are not just good games, they're the best in the genre. The GOAT. 4 player MK 64 with 3 other mates is still one of the best local multiplayer experiences available. MK Wii's got the tightest controls and some of the best tracks. MK Double Dash had the unique dual character gameplay.
Don't get me wrong, CTR is definitely top 3. But MK has gotten better with each release.
My vote would go to Diddy Kong Racing. The adventure mode rules.