There was no real competition to GTA3 though when it launched and even for a few years after until Vice City.
In my option, there has been no other game since Vice City which has achieved the same genius of in game humour, depth of character and gameplay and just sheer fun-ness of it. Vice City managed to retain all the entertainment of just hooning around and blatting people without consequences or feeling guilty that all the earlier GTA series had, but introduced enough depth to make you want to play through to 100% and keep you going back. Vice City was also the perfect size gaming environment/map.
San Andreas was a good game but started to introduce a few more serious elements which I felt detracted from the game after a while. It lost a bit of the charm that Vice City had, but the sheer size of map and depth gaming environments were amazing, especially for its time. I felt the SA map was almost too big with a lot of it being soulless and just space for the sake of it. Still awesome though.
GTA 4 was a huge let down for me. I felt it had lost the fun factor of the earlier games. A big part of this was perpetuated by the revised physics, improved graphics, added realism and more noticeable consequences which took away the fun of hooning around and blatting people.
The Just Cause games have been the best of the newer generation copies imo. Saints Row is good and SR2 is better than GTA4 for fun-gaming but let down by in-game physics.