Depends upon criteria.
Overall it has to be PS2 I think. It had broadest catalogue of games, so the console really did offer something for everyone. It has kids games, adult games, sport games, RPG, etc.
Gamecube was really nice but a little to one note for me. Xbox at that time for me represented a cheap PC for those that couldn't afford a decent PC - also I'll never forgive the console its part in ruining Dues Ex 2 and adversely affecting Thief 3.
Dreamcast never really got the chance to make an impression (at least from my point of view).
I guess in terms of simple metrics such as sales of console, dominance of brand, sales of games, etc. the PS2 obviously dwarfed everything else.
Probably worth noting however that last gen pretty much all my gaming was on PC as I bought the PS2 for my kids - although I did play ICO and SOTC (two of the best games ever) and Silent Hill 2, 3 & 4 on it (oh, and Farenheit or whatever it was called - interesting if flawed experiment and I'm curious what its development team do with heavy rain: hopefully they won't cover the actual game with large flashing button cues this time!).
Try to be reasonable... its easier than you think...
