GTA-III and Vice City were absolute gold, even with their wonky control system (by today's standards).
San Andreas was too big with not enough stuff to fill the dead space, and I really didn't like having to eat and workout, etc. I didn't play that one very long, but I remember having a blast with the cheats.
IV is the most forgettable to me by far. I remember liking the driving a lot, but getting too many annoying text messages for side stuff like entertaining my girlfriend. The city was pretty cool though.
V was great, IMO. I only played the single player once, but have put probably 300+ hours into Online. It's got plenty of problems, but I don't care. I still have a blast. I also don't think the game's makers were ready for Online's success. I'm expecting to see a much more refined online environment for the next game, possibly even the start of a permanent world that just continues to be added to as time goes on,