Well, game of the decade is completely opinionated and subjective, just like GOTY.
Plus, it's kinda hard to tell without knowing the next 10 years.
I hope they get it right in GTA VI, with all the technology in the next years they should be able to make the perfect GTA.
Depends on your definition of perfect GTA. To me, the perfect GTA would include being able to fly from Los Santoas to Liberty City to Vice City, etc. Next-gen can't do that. PC's now probably can't do that.
Technology CAN do that. It could have done that ten years ago or even more. Just look at the map size of Elder Scrolls Dagerfall.
The reason why Rockstar doesn't make GTA's that include too many cities is simply because that would take A LOT of work and A LOT of resources. And there's also the problem of disc space that was limited by Xbox on current gen - but even with that resolved now, they still don't want to spend years just making that huge map. They may one day (assuming they stay on the same engine) update the previous cities with new textures and some updated models, and then include them in one game, which would make their job easier and thus make that dream of all GTA cities in one game come true. But not before they've made HD versions of all of them.