I'd suggest IX, but it is a bit slow and a lot of the subtle nods to other games would go over your head. IX may be my favorite, but I recognize that it probably isn't the best one to start with. I say start with either VI or VII.
Anything before GTA3 is going to be radically different from the later games in the series. If you want to see how the series has grown and changed over the years then I suggest you start with wither 3 or Vice City. Or fuck it, start with arguably the best game in the series, San Andreas.
GTA V. I thought GTA IV was a step back for the series. Basically, it felt like they removed all the fun stuff and interesting characters and replaced it with bowling with Roman. It was all super forgettable.