Played all of them, and in order, mostly around the time they came out.
2 was the best of the 2D ones, though I found London to be on par with it.
3 was.... raw. It was very interesting for the novelty at the time, but it doesn't hold up all that well.
Vice City and San Andreas is where the series peaked.
Good gameplay, and still an actual focus on gameplay instead of glorified series/movie stuff.
4 was still quite decent.
5 was really just a glorified movie, gameplay played second fiddle. With the only decent way to make money being the stock market with pre-knowledge about story events... dumb, just dumb.
The worst part about it were the characters, though. Two completely unlikeable assholes/clowns, and Franklin.
Couldn't even finish it.
I have no doubt 6 will again just be of the glorified movie variety.
But maybe, just maybe, not all of the playable characters will be assholes... that would make me get it on a sale at least.