Super Mario World for me, but half the reason is nostalgia.
Fondest memories playing that game with my brother decades ago.
If I was new to the series I think Galaxy I and II would be my top picks. My only complaint about those games is that the levels are so damn linear. I don't want a full GTA style open world for Mario but say 8 huge themed worlds with 20+ starequivalent shinies to collect (among other things) in each area.