I say GTA V simply because of the online mode. GTA IV's online was very addictive and fun for me personally, especially considering the lack of headsets back then. Had some pretty good memories with it, but I don't think anyone can say GTA V's online isn't obviously the best online model Rockstar has done so far. As far as pure story goes, GTA IV takes it. I think they tried too much with the GTA V story and that made it hard for me to really get invested into each character's story. Had the game just focused on Michael and let us play as Franklin or Trevor on occasion, I think that would've made it better.