My personal favorite and most under rated game of this generation is Crackdown on Xbox 360... The most fun I had on my 360. The best sandbox game ever! The game had a few problems, but the fun factor was 10 out of 10.
The most overrated would be GTA4, it got perfect 10s everywhere, and im a huge fan of the ps2 games but this one was slow and boring and way too serious. I give it a 7 out of 10.