I've played 1, 2, the london one, III, VC, SA and 4.
I don't have enough experience with the first 3 I listed, but I have played them. So out of the others, for me it is 4.
I did play the one DLC for it which made it interesting with new weapons, but for some reason just killing things and evading the cops wasn't as fun in 4, and I don't know why.