MotoGP: I got hooked on this while beta testing Xbox Live.  The Xbox Live Beta disk only had a couple of tracks.  I promptly went out and purchased the full game.  This was fun and the application that got me hooked on Live.  There were some issues with backward drivers and glitchers, but they have largely gone away if you can find a session any more.  Almost a must have, but only if you have an original Xbox Live disk that came with the online version.

MotoGP2:  Bought this as it had the Xbox Live built-in, got rid of a bunch of the glitches, had nicer graphics, and took care of the backward drivers.  Still wasn't as much fun.

MotoGP3: Rented this and just couldn't find a compelling reason to purchase. The added Extreme Mode street tracks were very interesting, but decided to wait on the MotoGP 06 for the Xbox 360

MotoGP 06: My new favorite racing game.  All the good of MotoGPs 2 and 3 with more tracks and better graphics.  Yum.