Favorite system of the 3 current ones?
I personally prefer X-Box the most out of the 3. Reasons being first with sports games it is much more convienent to have the X-Box HD to save the games as opposed to having to have multiple PS2 memory cards due to the large data files that these require.
Then add in the fact that I would rather have the boost in graphics that comes with most multiplatform releases. The other day I picked up the GTA Trilogy pack for the X-Box even though I already own all 3 games for the PS2. While the graphics are not a drastic improvement over the PS2 version, I find they are enough that makes it a bit easier to look at the game. It has a slight drop more polish to them and the graphical effects look better. The only time I did not purchase a multiplatform game for the X-Box was Need For Speed: Hot Pursuit 2 because the reviews said that the PS2 version offered the best sense of speed.
Third, the 2D game support. While none of the current consoles have a ton of 2D games on it, X-Box is much more adequate at 2D performance than the PS2 is.
The support from Sega, namely Panzer Dragoon Saga and Shenmue 2 are aces for me as well.
X-Box Live would be a factor if I used it.
But this post is not meant to be a rip on the PS2 or Gamecube because I enjoy both systems as well. I just prefer the X-Box for various reasons to the other two.