Either the NES or N64...hard to choose between those two. I had the NES before the N64 came out, and I remember how fun it was to play as a kid. I had a crap-load of games for it too.
Super Mario bros 1, 2, 3 , Ninja Turtles, Kirbys Adventure, LOZ, etc. etc.
I don't know, there was a weird mysterious feeling to the N64 that is hard to explain. I guess it's something to do with making the jump from 2-dimensions to 3 or something. Still every game almost has that same feeling...as if there is a secret place waiting to be unlocked.