I've enjoyed all the previous GTA's equally, though SA's huge world was the most fun for me. Especially with some golden memories with SA:MP when playing it with fellow LFS teammates.
For GTA 5 to be perfect for me, it'd have the setting of VC, huge open world of SA (not just boring city scenery like IV), the physics and (maybe a bit more refined/optimized) the engine of GTA IV.