The answer to both questions for me is SMB (1). I beat it when I was 3.
The first SNES Mario game I beat was, of course, SMW (1)
N64: SM64
First game I beat with my feet: SMW2.
(I might try SMW (1) sometimes... of course, it will be hard with the time limit and vastly different hit system....)
Everyone has their own opinions, no matter how WRONG they are.