4 was so perfect. I always dreamt of a cheat or patch to allow walking.
American Wasteland was really good, combining the best of thug 1 and 2.
2 is iconic and has some good levels, but it's so repetitive and 3 fixed that.
Warning: Sometimes biased
http://i.imgur.com/V0x5fw8.jpg http://i.imgur.com/IOovUge.gif http://i.imgur.com/zw7bqPH.jpg