Wow that's a tough one. GTA III was my intro to the series and to open world gaming in general, I was just a kid and my dad would let my brother and I play and it changed my gaming life. San Andreas is still one of the coolest overall stories and the mechanics and side activities were groundbreaking (and until 6 were exclusive to SA) plus it was one of the best for cheat codes other than Vice City. I didn't play VC quite as much as the others but still had my fair share of fun. GTA IV may have been the first time I felt like I was playing the greatest game of all time. Side activities like going to comedy shows and the ultra realistic ragdoll physics made for endless hours of fun and it may have been one of the first online games I ever played. When 5 came out it literally changed the game when it came to open world expectations. The multi-character story and especially the introduction of first-person were absolutely amazing. But for me personally, just because of the story and overall aesthetic, The Lost and Damned has always been my favorite of all of the GTA series. Having a biker gang in 5 online just made me long for the days of riding as Johnny at the front of the pack.