Sonic & Knuckles + Super Mario World are my joint fave platformers of all time.
However, overall i have enjoyed more Sonic games than i have Mario ones (these two are in a different league to the rest when on song IMO).
Before people say theres been no good Sonic games in the past 15-20 years, i suggest you go play Sonic Advance 1,2&3, Sonic Rush & the 2D levels on Sonic Generations (HD twins version, not the 3DS one)
+1 for Sonic.