It's a tough call between PS1 and X-Box. Whilst the PS2 can of course play PS1 games, the PS2 itself hasn't had as many I consider really awesome, classic games as the PS1 did.

The X-Box, on the other hand, has proven itself to be a robust console with a wide selection of games (Could do with more RPGs though), has the best versions of any cross-platform games, the best controller (Well, the original large one was, the new smaller ones aren't so hot), is the most reliable, and generally I have few complaints.

Of course I could be really pretentious and say Colecovision xD