Shameless re-post, but it has more relevance here than where I originally posted it at least.
There really is no "better" game in the series. They should all be taken as their own experiences, but unfortunately people can't leave the past where it belongs: in the past.
GTA 1 & 2 - Never finished them, but the top-down wasn't for me.
GTA III - I loved the atmosphere in this game, not sure what it was but it was great. Shooting system was horrid. No swimming was a bitch and the camera needed a lot of work. 8/10.
GTA Vice City - Still no swimming, still the same awful shooting system. Great soundtrack and it nailed the 80's vibe. I liked the celebrity voice-overs like William Fichtner as Ken Rosenburg and Dennis Hopper as Steve Scott. I used to love to mess around with the flying car cheat. 8/10.
GTA San Andreas - A massive and diverse map, but with a rather lifeless environment. Another home-run soundtrack, and now swimming was finally in. Planes were extremely fun to fool around in. Story was mediocre and felt rushed, all over the place and losing focus. Customization was great for clothing and haircuts. It had a better shooting system, but it's pretty bad going back to it today. 9/10
GTA Liberty City Stories - The great atmosphere from GTA III returns and I loved the almost all-original, in-house soundtrack. Definitely had a better story than GTA III. I was disappointed that they didn't implement swimming. And hell, it was a full-fledged GTA on the go. 9/10.
GTA Vice City Stories - Great Soundtrack, and FINALLY swimming in Vice City. Aircraft was lame because the altitude ceiling was ridiculously low. A mediocre story. I loved the business empire aspect. It ran pretty bad though, horrible frame dips and bad pop-in. 8/10
GTA IV - Definitely had the best story in the series, I loved Niko, Packie, Little Jacob and Dwayne. Some of the atmosphere from the PS2-era Liberty City found it's way into the new one. A MUCH better combat system than any GTA game up to that point. So much unbelievable level of detail. I loved the internet and Republican Space Rangers. The city felt real, like a place you could actually find yourself living in, just because of how detailed the enivronment was. My only faults are the mixed-bag soundtrack and piles of useless cash. 10/10
GTA IV TLAD - A really mediocre story and the biker theme wasn't for me. The grain filter was stupid and I always had it off. The new weapons were great. The dialogue was well-written. 7/10
GTA Chinatown Wars - Probably my least favorite. It's pretty amazing how they managed to fit most of GTA IV's map onto the DS. The story was extremely uninteresting and of course the lack of any decent music was a huge blow. I wasn't fond of the "arcade" style that the game had. 6/10
GTA TBOGT - It certainly brought back some of the over-the-top stuff from San Andreas, like parachutes and action-packed missions. The new weapons were fantastic and the new songs were great. I wasn't big on the story or Luis, it all felt too forced, like R* was trying to play catch-up with Saints Row 2, and yes I really did just say that. Mission replays were a fantastic addition to the series. 9/10
GTA V - I'm still letting it all sink in, after all, it's only been out for a month now and it takes time to fully-appreciate GTA games (I STILL find things I didn't know about in IV). I'll start with the main characters. Franklin is definitely the only "sane" one of the group, sort of like Niko in GTA IV. I thought he was cool, but it got tiresome listening to him complain, especially to Lamar. Michael is probably my favorite, a real chill guy living the everyman, blue-collar lifestyle and then makes his way back into the criminal underworld. I really loved the family aspect because you don't get much of that in video games. Steven Ogg gives the best performance as Trevor. It's really hard to get me to laugh, but Trevor managed to make me do it on several occasions. I loved the scene where he was arguing with Floyd and his girlfriend, and out of nowhere Trevor asks them to marry him. The map is great, but I wish there were more small towns, a criticism I had of RDR also. The soundtrack this time around is another mixed bag, but I love Space, Radio Mirror Park and LSRR. The planes are definitely better than in San Andreas, flying style and selection. The shooting system isn't as good as IV's and I miss being able to see enemy health. The missions and side activities are a lot of fun, especially base jumping with that godly track that goes with it.