From what I've played:
GTA2 - 7/10. It is a good game, however I can't enjoy it fully because of 3D-era games.
GTA III - 8/10. This is a great game, a setting (dark Liberty City with all the gang hostility) is amazing, and a storyline is good too, though it could have been better. The only thing that really bothers me is difficulty. Not that it's hard as hell, but it can take some time for a newbie to pass Portland.
GTA Vice City - 8/10. Atmosphere is what makes this game, not the plot. Also has a badass V-Rock radio and Tommy Vercetti.
GTA San Andreas - 10/10. The best game in the whole franchise (as for me). The setting was great, and the storyline, even though it consists of random events mostly (especially when you come to desert and Las venturas), is awesome. Other stuff, such as secondary missions (although they are the same from VC and III mostly) and other features, supplements the game.
GTA IV - 7/10. Yep, I didn't enjoy this one. It only got me near the end, when Niko started working for Jimmy. Other than that, it was more of a 'meh' game.
GTA TLaD - 9/10 - This one is just great. The theme, the setting (which is more of a III's LC than of IV's LC), the story - everything is very well done. The only thing it lacks is a bigger storyline, but I guess that for a DLC expansion pack it is great.
GTA TBoGT - 5/10. Can't even give this one a good score. This is a shame for Rockstar. I can't tell what exactly is bad in TBoGT because the whole thing is bad.
GTA Advance, GTA CTW - Can't really rate this one because I played them on emulators.