Sonic for me, though it's partly because of nostalgia. Played them to death as a kid, and will still pick up my genesis and play them. Sonic 1, 2, 3, Sonic and Knuckles, and Sonic CD are just pure fun. I like Mario, but it's just not the same as my love for Sonic the Hedgehog. There's not as much "stuff" in Sonic games like power ups or secret exits, but I always loved how the special stages for chaos emeralds always brought in a completely new play mechanic and spiced things up a lot.
Neither have aged badly, and either are very great reasons for the VC's existence to experience the one you missed out on.
