Haven't played very much GTA, but it's one of those rare series where most of the games are very good. Most of my playtime has gone into Vice City and San Andreas, but I've played 1, 2, and 5 a good bit too. That said, the original 2 aren't really comparable to the 3D entries, lol.
It's funny, I've been playing through Vice City the past couple of weeks and I think that I prefer it over San Andreas, BUT San Andreas has more polish and is easier to play today. That said, I'll have to actually load up San Andreas since it's been a few years, to compare side by side. I just really like the 80s vibe of Vice City, and that the world is compact but meaningful. If Vice City had the polish of San Andreas, it would be looked upon much more fondly than it already is.
Hot take, but things like GTA5 don't hit the same for me because the world is just SO BIG. Something like Vice City, you really learn the world in just a few hours, but in GTA5 it just felt extremely overwhelming.
All that said, I need to try the other mainline GTA games, plus VCS and LBS, and whatever else I'm forgetting.