San Andreas - Starting out, I spent a couple hours just criming about town trying to earn enough money for an afro. This was far more entertaining than it should be. The scope it epic, the story well played (damn you Smoke!), the environment sprawling. Is it just more, more, more? Well yes, but it's also way more fun. Get a couple homies to ride with, jack a lowrider, and do drive-by's. Really, the criminal aspect is played up to it's fullest, most entertaining quotient here. And it's the >crime< that people like about GTA, sandbox be damned.
GTA3 - Hasn't aged well, maybe, but damned if I'm not playing through it right now, and it still entertains mightily. I attribute this to two things: the environmental progression (opening up first the urban core and then the suburbs somehow feels more rewarding than any other GTA) and the fake pop music (which absolutely kills me, and I wish they still did). It's like the perfect balance between the original GTA's awesome arcade sensibility and the emergent sandbox, 'real-world' idea.
IV - By far the most impressive, and yet not as fun (save for the multiplayer, which >can< be transcendent with the right group of people, with the right settings). Jacking a helicopter and being amazed by the digital creation of Liberty City is obviously on another level, but when it comes down to the gameplay, it seems a bit too forced. I don't know, I must be getting old because this is a problem I have with a lot of 'next-gen' games: they just don't seem as much fun to me. I like arcade style mayhem, and this one gives you simulation-level stress. I'd rather cause chaos in a firetruck than pick my buddy up for dinner any day. A personal perspective, to be sure, and I both appreciate and enjoy the ambition behind the game. I just wish it was more fun (and I'm sure as the next-gen iteration advance, we'll see the chaos seep back into the series).
GTA - GOURANJAAAA! Straightforward, simple, entertaining as hell. I wish all the games offered a zoomed out overheard perspective.
Vice City - While it has some of the most entertaining scripted missions, I just didn't like fucking around in the flat, mostly linear city of Vice City, to the point that I never got to the second half of the game. This, evidently, is when it becomes entirely worthwhile, so I'm working on that at the moment. We shall see.
GTA 2 - not played. But the idea of a GTA set in the future is very appealing to me, and something I hope DMA/Rockstar considers in the future.