The original GTA would be top for me - not only did I spend forever playing it, it introduced me to the idea of mods, which ended up directing me to a career as a software engineer that I thoroughly enjoy. It's also got the best soundtrack, with not a single radio station anything other than perfect.
Second would be GTA3, because it was a huge leap in technology and has by far the most nostalgic setting for me. Third would be GTA4, again because of the impact that the new engine had, as well as the great atmosphere and story, slippery driving, and the ability to free roam the city co-op using the multiplayer mode.
Of the rest, SA is still at least a 9/10 game as it really mastered the PS2-era GTA and added so much depth to the features, albeit some of it a bit unnecessary. GTA5 is good, but a bit of a step back from 4, redeemed by GTA Online which can basically be played as a sandbox GTA with co-op missions. GTA2, London 69 and London 61 are sort of GTA1 but weirder - 2 had some nice advances but is overshadowed by the gap to 3. VC has a great setting and stupendous soundtrack, but it's probably one of my least favourite to play with all of the drawbacks of 3 plus a worse map layout, and the property ownership mechanic that just broke up the flow of the storyline. Chinatown Wars is okay, then LCS and VCS I didn't play much as it was awkward using a PSP for complex action games, and by the time the PS2 back-ports came out they just looked outdated.