GTA IV, no question at all. While more custimization on a few things would have been very much appreciated, virtually everything else they added in was for the better and made it a brilliant experience. While a lot more of the radio was forgettable, they had the best classic rock station in any of the series, so it doesn't matter too much, and the improvements they had to both the shooting and driving were perfect, especially when put into free roam.
I honestly don't get people who moan about the cars suddenly being crap though. The majority of them felt different from one another, something that couldn't really be said for earlier installments, and had a bit of a learning curve to them. Once you started to get the feel of it though, it nailed the spot between a set of mechanics that allow for several options and one that isn't too complicated that you need to know it inside out in order to do it right. All people need is a bit of practice, and to not hold down the trigger all the time, especially when turning, and they'll realize that it's actually a very solid system that blows most others out of the water, especially Saints Row 2.