V is by the best in the series, as well as the best open-world game ever made. For me, however, I'd say that IV is my favorite. IV has a top-notch narrative, set in a highly atmospheric gamespace, with some of the best acting, writing, direction, and mission structure ever put into a game. And despite what people say, the driving is phenomenally fun. Hell, the physics alone are fun to play around with, and rampage.