San Andreas was just San Andreas. IV was the one with Nico in Liberty City. It got some good reviews at first, but it's been kind of blasted over time as it hasn't held up as well as the others. The characters and story were weak, a lot of features/vehicles were removed from previous games, it just wasn't Rockstar's best work.
I'm torn between San Andreas and Vice City as my favorite. Vice City had the whole Scarface thing going for it...cool cars, great setting, good soundtrack, and even the talk radio was hilarious. "All the bunnies are stabbing each other!" I loved that bit. San Andreas had a huge map, tons of vehicles with the addition of some real aircraft, colorful characters, gang wars and turf, three very different cities to explore, and a ton of shit to do. I prefer Vice City's setting, but would have loved some features/vehicles from San Andreas.
As far as the new GTA V, it's pretty good. I'd put it at #3 or #4 over all the main titles (I haven't played some of the obscure ones like "Ballad of Gay Tony" or "Liberty City Stories"). I still can't believe they haven't released Online Heists for multiplayer. I was looking forward to that the most...supposed to be out in the next month or two, but I'm not holding my breath. A lot of players have moved on at this point.