IMO GTA V on CG without a doubt. Why?
The map is amazingly detailed and extremely diverse and beautiful. There's beaches, hills, a wind farm, swamps, a military base, rivers, caves, a big sea, lots of unpaved roads, huge mountains, towns, canyons, small forests, an ocean floor, small villages, a quarry, an offroad track, etc. Best map in the series.
The amount of different vehicles is just insane. Great to see the planes making a return, as well as the bicycles, adding to that the great driving physics that are a slight improvement over IV and the massive car customization options that is a huge improvement over SA.
The story has flaws but is still pretty good and among the three best ones in the series, behind IV and VC's. One thing that V's story has over others in the series is that it feels more "personal". What I mean with this is for example that many missions aren't you working for gang bosses or corrupt cops, yes V has this too but it also has many missions that involve personal things, like Trevor Philips Enterprises or Michael's family. Michael loves movies so it's great to see we get to "work" on one, something he always wanted to do.
The player customization is a huge improvement over the precedessor, even though I don't care much for the tattoo options. I really like the weapon customization that has been added for the first time, with the CG versions the attachable scopes get more useful, though the best attachment is obviously the extended clips. Great to see that the minigun made a return and then there's the hatchet which is one of the best melee weapons in the series.
I like the way how everything kind of feels connected together, like some of the story and strangers & freaks events that get mentioned on the internet or how Steve Haines hosts a show on the TV, past GTA's already did that pretty good but V made it even better. And to add to this, there's actually a back story to pretty much all of the collectibles in V. IV did have this too but it was just a minor detail and then again the flying rats are the worst collectibles in the series. Only thing what 3D era collectibles made better were the rewards. Overall the collectibles are more intersting in V.
With CG V has one of the best soundtracks in the series. LSRR, Non Stop Pop FM and Radio Mirror Park are my favorite stations. The score was a great addition, adding more atmosphere and intensity to the missions.
I think I went a bit too much into detail and forgot about one obvious thing, the graphics of course. Anyway, second place goes to IV and third place to SA.