I honestly cannot decide. Of all of the options above in which I own.. Which are the N64, Atari 5600, SNES, and Playstation, I honestly cannot pick just one.. So I will talk about all of them, and pick after.
N64: I've had a lot of fun with my N64. It has some of the best game's I've ever played. Of course including Majora's Mask, Mario 64, Mario Kart 64, Pokemon Stadium, Lego Racers, and Kirby: Crystal Shards.. Just to name a few. I'd have to say recently I've had the most fun with Mario Kart 64. My friend and I play it hours at a time, and even though we've got gold on all of the cups on 50cc, 100cc, and 150cc, we still play it often, and have so much fun.
Atari 5600: The thing I enjoy most about this is all of the classic arcade games I can play. It's so much more convenient, and fun then playing them on an arcade pack for the Gamecube, or playing them on the internet. Though my dad gets the most use out of the thing by far..
SNES: I agree with most everything everyone else has said. The SNES is another classic with great games, and great memories. Super Mario Kart, A Link to the Past, Super Mario World, and Earthbound to name a few. I can't say that my friends enjoy playing my SNES quite as much as my '64, but it's still excellent.
Playstation: There aren't quite as many classics on this one, but it's still great. Metal Gear, Crash Bandicoot and Final Fantasy. I especially have fun playing Crash Bandicoot on this thing. I've spent so much time. That game really is quite enjoyable..
All and all, I hate to say it, but I have the most fun with my N64. Mostly because my friends have the most fun with it, and there really are some great games on there to play with friends.