SNES is probably easily going to be it, but I personally never had one so I just don't have the same attachment/nostalgic value for it, even though I've played a lot of the games.
GBA and it's various alternations is a good choice. It had a long life, huge library of games, and it's friggin' portable! It was an amazing improvement over the GB/GBC.
However, I'd have to say PS2. I love my Gamecube, but PS2 just had too much going for it.