Well in terms of an error free game, 08 was the worst. In terms of technology, control depth, and dynamic gameplay, 08 was the best. And 08 has some of the most exciting moves and gameplay in any NCAA game. The lateral feature is different in 08 than in 2005 or 06. There's a lot of things that make 08 a blast. But it's one of those games where you are going to run into bugs if you really mess with the settings. The bugs don't ruin the game as there are ways around then. A lot of times your defense has to rely on manual control, where in 2005 you could rely on your CPU teammates on defense. The receiver direct gameplay feature is one of the most innovate features in any game and it's in 08 on original box. So if you nitpick 08 on original xbox too closely, you'll find things that annoy you. But if you go into the game with an open mind, and just want to explore playbooks then it's great.
Oh and one other thing that 08 on Original XBox has that 2005 or 06 doesn't have is user stats. 08 keeps track of all the things that you do with user control. So if you play a long game and at the end of the game you can see how much of the tackles, knockdowns, etc you did with your own player. Just a nice little touch.
The playbooks are very expansive in 08 on original xbox. Most plays ever in any NCAA game. But there are some glitches when you start editing playbooks in 08 that wasn't in earlier games, so beware. It comes down to fact that in 08 on original xbox, there are more plays in some formations in team-specific playbook then you can find in custom playbooks. So you can find some team-specific trick plays in each teams individual playbook, more team-specific trick plays than any other NCAA game.
Things like that make 08 worth owning. And i'm sure the PS2 version is good. But 08 was the last NCAA game on origianl xbox, and graphically it is a huge improvement from 07. The lighting is spectacular. Just a great game all around with unlimited potential, hindered only by developer tight schedules due to the takeover of 360 / PS3 football gaming by EA.
If for nothing else, PS2 or Original Xbox owners owe it to themselves to play NCAA Football 08 just to see how close EA was to football perfection with that engine before they abandoned it for a new engine on the other systems.