Best in the series. One of the best in the genre.
As a veteran, however, I can say that the bare mechanics of the game take a little getting used to. As I've said before, the action is fast – paced, a first for the series. The new setup is more streamlined than in the past games, so while it may be easier to navigate, again it takes a while to get used to it. While veterans of the series might not feel entirely comfortable with the new engine, it quickly grows on you and you familiarize yourself with the gameplay pretty quickly. For a newcomer into the AC mythology, it's only that much easier.
This game also has a replay value consistent with the series. After the first playthrough, you've only completed about 60% of the game. There's still the "hard mode" that needs to be beaten and a bunch of parts and weapons that you need to get. It will be a long time before your completion rate is 100%.
To reiterate, AC4 is definitely the best game in the series, and one of the best in the genre. It's definitely worth having in your repertoire.