Yes, it is.
It does nothing revolutionary, it merely does things better than they have (for the most part) been done. But one would hope that would be the case. It still suffers from all the standard Rockstar issues (dragging a solid 15-20 hour game out to 40, lengthy travel sections that serve no purpose but to stretch time, somewhat... bad multiplayer, etc.) and it really is basically GTA4 with horses, down to the basic structure and progression of the story, right down to the "three islands" structure of the world map.
Don't get me wrong, it's a good game, and I had a great deal of fun playing it as much as I did.
But it doesn't deserve to be remembered as one of the best ever, which it seems to be. It's a progression, a refinement, not a next step.