GTA San Andreas has to be. Great story line, my type of graphics and gamemode. City is lovely with all the buildings and the type of places. The Multiplayer of GTA SA on PC is even better! 10/10
They should involve some unique stuff which other games haven't based yet.
It might be hard but make a lot more buildings enterable by walking in. Activities to do like sports,gaming and lifely hobbies. Surfing, Basketball, Jet Ski, Parachuting etc.