I enjoyed the hell outta GTA IV, and still play online with friends. It was a living city, and as Kaker said earlier, the driving was great! And the physics? You can't tell me that was one of the greatest improvements in the GTA series. I enjoyed the customization of San Andreas, but GTA IV had fun characters, fun story, and a great engine.
http://kotaku.com/5855980/grand-theft-auto-v-will-be-the-series-biggest-game-rockstar-says?tag=gtav
Kotaku has a whole section on GTA V, check it out!