I'll give 'em all a rating.
The PS2 gets a nine. It falters due to having a far weaker online experience than the Xbox (I assume. I didn't play Xbox during the 6th gen). I had a ton of amazing games from Final Fantasy X to MGS3. Simply the obvious choice for that gen.
The Gamecube gets an eight. It was beloved by me. It had some great games in its own right. It sorta petered out at the end while the PS2 was still getting content but in its prime, it was great fun. I still have most of my Gamecube library as well as most of my PS2 library and, looking at the two, there's still more Gamecube games I'd want to play today. Some of those games were timeless, though at the time, the PS2 library decimated it. No online experience at all and multi-plats were sometimes better and sometimes, much much worse (ie: True Crime or Splinter Cell).
The Xbox 360 gets an eight as well. I didn't own one but I experienced many Xbox games. It had a ton of exclusives, though not as many as the PS2. It also had the best online and, in my opinion, the best graphics. Multi-Plats on Xbox actually had to be altered to run on Gamecube and PS2. Splinter Cell, for example, had to have its levels changed because they were too open on the Xbox. Xbox falters when it comes to the timeless classics. The Xbox games I played at the time (Brute Force, Dreamfall, etc.) relied on graphics instead of gameplay. Still, many games hold up--Conker, Halo, Jade Empire.
In the end, you just couldn't go wrong with any console during the 6th gen. That's where everyone besides Sony suffered. Every console had great content but PS2 was really all you needed. There just wasn't enough time and money for everything else.
