Definitely Mega Man. I'm a big fan of the series, and seeing Capcom neglecting to make more Mega Man games for the past few years just saddens me.
Bloody Roar is another one I would like to see make a come back. It's a pretty underrated fighting game series.
Here's some levels I created in the past:
76AB-0000-0032-55F3
FCB0-0000-0037-D8F4
F6AB-0000-0059-3FDE
A special mention goes to this one though: 2F67-0000-01FB-3416
That level is a recreation of SMB World 1-1 in the SMB3 format.
I prefer the New Super Mario Bros. series over the old 2D Mario games. Don't get me wrong, the original Mario games were revolutionary, but when it comes to the core gameplay, I thought the NSMB series took what made those games great, and made it even better.