Super Mario 64.
Most are 'excellent', others are merely 'good'. Personally I feel Yoshi's Island is heavily underrated while Super Mario World is probably the most overrated game in the series. 64 wins easily in my book, it's pretty much the perfect game. Yoshi's Island is a good second, the best 2D side-scroller ever made.
I barely played Super Mario Bros. 2, so can't say much about it (and I won't include it in my list here). If you'd include the NSMB series, they would be below Super Mario World (NSMB>NSMB2>NSMBWii), with the exception of New Super Mario Bros. U, which would be at number 9 knocking Super Mario World down a spot. I also have Super Mario Land 1 and 2 on GameBoy, which I'd rank even below NSMB. Fun, but didn't care for them much. In fact, I think the best Mario on GameBoy is Super Mario Deluxe, which is an enhanced port of Super Mario Bros. from the NES. The 'excellence' comment stops after Galaxy 2, after that, they're 'good'.
1. Super Mario 64
2. Super Mario World 2: Yoshi's Island
3. Super Mario Bros. 3
4. Super Mario Galaxy
5. Super Mario Galaxy 2
6. Super Mario 3D Land
7. Super Mario Bros.
8. Super Mario Sunshine
9. Super Mario World
Can't wait to find out where Super Mario 3D World will fall !