Well, if I were to consider hours played on each system than my Dreamcast would win hands down. I own 2 DCs and had many happy hours gaming on them.
Second runner up would be Gamecube. I've actually run my cube into the ground from so many hours playing on it, and will probably be replacing it soon. However, half of that time was spent playing Phantasy Star Online, which was basically a port of a Dreamcast game.
Out of all my systems, my XBOX has gotten the least amount of playtime on it. I just bought a ps2 6 months ago, and so far my ps2 has gotten more game time than my XBOX which I've had for 2 years. There are just simply more games that I am interested in playing on my ps2 than my XBOX. However, I've taken neither systems online yet, which is going to change in about another month once I finish working out all the bugs on my new high speed net connection. Then we'll see which system really gets the most use.