Not surprising for those who know, but historically GTA is not really my kind of game.
My introduction to the series was over a long weekend in subzero conditions with a couple of friends and not much to do. One of them had a copy of Vice City. I played for probably 12 hours straight, just cruising around stealing vehicles and such. I had enjoyed Driver for the similar free roam feel the cities had, and this seemed like a natural extension of that; with way more freedom. For the next year or so, my only exposure was occasionally playing Vice City aimlessly, with very little exposure to the actual storyline and gameplay.
The first GTA that I bought was San Andreas, in February 2005. I bought it at the same time as GT4 upon the latter's release. That game is still my favourite in regards to storyline, gameplay, and overall experience. IMO, San Andreas was much further ahead of its time upon release than any other GTA. I also consider that game the pinnacle for my personal game playing style, which is far more sim oriented than anything else. I could ramble on for hours, and days, about how I still lament the fact every time some new potential feature is discussed I can usually say "Yeah, they had that over a decade ago in San Andreas." I still feel that despite the huge indisputable leaps and bounds that have been made in gaming technology, and the use of that technology in modelling the most living and breathing environment of the series; there was still a lot more to do in San Andreas. For a player like me, that is(read: non-combat/leisure activities.)
My playing time with GTA IV was limited, as despite being intimately familiar with the region it did not appeal to me. I did not even buy IV until 2013, and played it for less than a month. Most of my time was spending hanging round Flushing Meadows. The lack of fixed wing aircraft totally killed the appeal of IV for me, and the perpetual March climate did the city no aesthetic favours. IV wasn't a game of signifcant leisure, nor did the environment capture my attention like San Andreas. I played San Andreas fairly regularly(as regular as an old game is played; put it down for weeks, pick it up for a bit, etc) right up until the release of V. I even played San Andreas a few times in the early days of V, and I can still appreciate that game for what it is.