Of the classical formats:
1. Yoshi's Island
2. Super Mario World
3. Mario Sunshine
4. Mario Bros. 3
5. Mario 64
6. Mario Bros.
7. Mario Bros. 2
I'm going to go with the most addictive Mario game, Yoshi's Island. Something about that game just rules (as well as Super Eagle mode).
Super Mario World is the most rewarding game, best secrets, and excellent gameplay.
Sunshine is just waaaay better than Mario 64, I'm sorry. M64 was good, but I found it to be the most boring Mario game there is, along side Mario Bros. 2