I own all three and for me the hierarchy goes:
Game Cube
Its lack of loading time, quality of game-play, and they easy to transport nature of this system makes it an easy choice for no.1. From the Legend of Zelda series all the way down to good ol Mario (yes I said it,) as well as exclusives such as Eternal Darkness (

) and the superior version of Resident Evil 4 make a clear cut winner for me.
X-Box
Easy No.2 here. The hardware on this machine is amazing. The built in hard drive is an achievement in gaming. Most console developers would have buy a hundred memory cards, yet the X-box gives you more than enough to enjoy though its entire life span. It has a plethora of third party titles, though it lack in first party. Still successes from so many of its games speak for themselves, but the X-Box only gets 2nd by default.
PS2
Its a shame Sony puts the most money into getting quality exclusive titles because quite frankly their system is a piece of shit. Thanks to them buying off most developers, they have put a bearhug on the market as most of the "must haves" are exclusively PS2 only. This is sad because graphically its way behind the times. The quality of their on-line support is way behind the times (at least they have on-line support.) From the number of bugs in the system to the lack of longevity this system gets the

from. If it weren't for the GTA's and such of that nature I wouldn't have the PS2. Though the PS3 is looking to be the be all end all, the PS2 comes in dead last for the current gen, from where I'm sitting.