If we only count physical sales per-platform, perhaps. But there were just way too many ports for this to ever be a fair fight (for a physical game).

I still find it highly overrated because the environments are boringly realistic and 99% of the characters are unlikable.

I think GTA 6 won't do it for sure. Expections will be much too high due to hype from fans who waited 12+ years for it to release.

I think the best hope is if GTA VI is a launcher for GTA Online (duh) and also a future GTA VII. 10 or so years from now, GTA VII will just be paid DLC to the existing copies of GTA VI. If that's the case, then I do think GTA VI will outsell GTA V. It would have two generations of releases at that point and a PC version.

Otherwise, it's a tall order. GTA VI is highly likely to be the fastest selling in the series at first due to the long wait. But long-term, I'm skeptical of it having GTA V's legs. 

There are Tetris games that are very different from eachother and not the same game. If you want to compare like that you have to go to franchises. These are the numbers.

I doubt VI will beat V. I doubt they'll wait that long again to bring out VII.

V launched right around the launch of the PS4 right? So it got lots of cross gen buys from the gen that was ending and was selling all through the next gen, and through half or more of this gen simply because it is taking so long for VI to come out. Probably people who triple dipped on it, and it was basically a brand new game when PS4 launched so GTA didn't really skip PS4. Meanwhile with VI coming out with probably like 3 years left in the current gen, it doesn't get two gens to sell to from launch, and I doubt GTA will entirely skip next gen so I think GTA VII will likely come out late next gen, giving VI only perhaps only 8 or 9 years to sell before the sequel rather than the 12 years V has had.

The insane popularity of the franchise always surprises me. I remember seeing GTA III for the first time and thinking it was sooo cool. Then the next year my roommate got Vice City and I thought it was a lot of fun for two weeks then it just got real boring. Later on I picked up GTA III for a few bucks, thinking it'd feel new since it wasn't Vice City, but it felt the exact same, was bored in like 10 minutes and turned it off. Never bothered to try the series again. Of course maybe the later games are a lot better, but at least those PS2 games were the type of games that were a lot of fun until you got to the point where you'd done all the fun stuff over and over and realized that's all there was to the game loop, just super boring after that. But obviously many many millions of people keep playing it despite that.

Yes, Concord.... oh wait

Will GTAVI get you a PS5 and PS6 copy in one, as well as XB 'play anywhere' copy, or will it get the $10 PS6 upgrade treatment? If so it will never come close to GTAV, otherwise if Rockstar just follows the same formula as before then it could certainly eventually take the crown.