1. Vice City - the best all-around GTA for its time, it was good in nearly every department, it had a simple and straightforward story that amazingly was very interesting and engrossing, constant action-packed sequence of events, memorable characters, soundtrack, and atmosphere, a stunning re-recreation of 1980s Miami, and revolutionary, groundbreaking gameplay features like empire building, interactive and enterable interiors, changing outfits, seen for the first time in GTA. Quite simply put, it was the complete GTA game.
2. San Andreas - still technically the best GTA in terms of gameplay features. Didn't have great graphics, but it brought more innovative features like gang recruitment, gang wars, respect system, gym, bodybuilding, gambling etc, and it still has the best map GTA map ever (3 different cities, each with a unique character, numerous small towns, varied natural environments - still no GTA map topped it to this day), it offered tons of action, activities, interesting and fun places to explore, which cemented it's huge replayablilty value. Not the best story or set of characters in the series, but still good enough for a very memorable single player experience.
3. GTA IV&EFLC - IV alone was initially a bit of a disappointment for me, until more playthroughs finally made realise what a gem of a game it really is. Despite a significant number of flaws and shortcomings (some of them important), it still managed to really shine in its own unique way - the best NYC re-creation in gaming ever, the best story in the GTA series, probably the best protagonist in the series, and it successfully took GTA into the HD era. It was stunning showcase of a very realistic and darker take on GTA's trademark open-world crime drama and action. I prefer TBOGT over TLAD but overall, EFLC made up for many of IV's cons, enough for the whole IV saga to be ranked above the technically superior V.
4. GTA III - the game that started it all for me. The game that made me the hardcore GTA fan that I am today. The first ever game of it's kind, 3D, free-roaming, open-world crime action game revolving around gangsters - a game had always dreamed of long before GTA even came out ! The only reason it comes 4th is simply because it was lacking many later features that made the GTA in-game world feel more complete, but as the first game of it's of kind, you can never be harsh on it for that, and that reason alone guarantees it's legendary status in gaming. Still has the best graphics in the III-era too.
5. GTA V - a most stunning looking game with the most spectacularly detailed graphics ever seen in the GTA series, a map of an impressive size and variety of environments, and it did very well to bring back some popular features missing in predecessor IV. A lot of explosive, action-packed moments in the gameplay too, but despite all these very good points, it was quite a disappointing, experience based on the huge pre-release hype it had generated for many reasons - poorly structured and largely boring story, weak characters, huge map that went to waste, still lacked many important and popular features, and in the end failed to live up to its great potential. I still enjoyed playing it nevertheless, but it remains the my least favourite of all the main GTA titles.
Note for V : Thankfully the many great mods available for V are what keep me playing it, and they certainly improve the experience multiple times over.
* I will not discuss LCS or VCS as I did not play any of them, weren't really too interested in them either. I've only ever played the main GTA titles and their DLCs. I will also not include the top-down GTAs or Advance because I have never any of played them and had no desire to, and my interest in GTA started with III.