Mario Bros 3. Where Nintendo really got the hand of giving each level its own gimmick and identity. The amount of variety they squeezed into a game with such limited mechanics is pretty astounding.
Rol goes through all the trouble to set up the poll, and then some clown spits on that work by picking New Super Mario Bros. 2. Some people have zero shame...
Anyways, I voted for SMB3, for the many reasons already shared. My runner ups were the console New SMB games, both of which were pretty good. I actually liked New Super Luigi U as well, but it felt too different to really make the Top 5. Still better than the handheld games, though.
I'm more concerned with the masochist who chose lost levels. NSMB2 wasn't inspired but at least it didn't actively try to punish you for playing it. And coin rush was pretty fun. Although why they decided to make coins the focus of the game and then not give you anything to do with all of the coins you collect is a mystery that I will never solve.