In single player:
Vice City > San Andreas > IV
All good games but vice city has more style and you don't have to take fucking long road trips for one stupid mission, and IV has a good story, but the missions are very repetitive. The best mission of those 3 grand theft auto's is IV's bank robbery mission for me tho :p
In multiplayer you can't rly compare tbh, IV and SA:MP are very different, because in IV there aren't dedicated servers (yet) and whether you prefer VC:MP or SA:MP is a matter of tastes I suppose, but SA:MP rules the player count for now.