I'm going to have to side with Del over Pike and MILF (wait, what!?) on this one. I love Sonic 2 as much as the next person. I enjoy Sonic 3 though it's not as good as Sonic 2, and I really never played enough of Sonic & Knuckles to completely weigh in but I remember not liking it nearly as much as 2 and 3.
But 8 and 16-bit Mario titles stab Sonic in the face and dance on the corpse. The original Super Mario Bros. is pure platforming bliss. Mario 3 adds immeasurably to the base the first one established and basically redefined the genre (no small feat when Super Mario Bros. basically defined the genre for many people a few years earlier). And Super Mario World and Yoshi's Island? There is no comparison there really.
I'll give Sonic some extra credit though because I loved Sonic Adventure to death. Far more than I should probably. Every 3D Sonic may have sucked after that, but it was an amazing smurfing game.