AC:R is one of the better games in the series so far, having played the three prior in chronological order. It refines many of the rough edges see in AC:B through the improved combat system and stealth detection. However by far the biggest improvement in my opinion is the brotherhood system, which was finally done right.
The gameplay is the best it has ever been, and the 16th century Constantinople setting is both a pleasure to look at and free run through. There are a couple of things out of the game from previous installments, such as horses and flying machines, but they have been replaced with equally interesting elements, although I felt that they sometimes derail the flow of the story too much. The story is excellent, far more engaging it's predecessor, but falls short of the masterfully segmented events of the second game. I think it's still worth the pick up before AC3 but isn't crucial to your enjoyment or understanding of the series.
Overall I would say AC2>AC:R>AC:B>AC1. The game gets an 8.5/10 from me, with AC2 being a 9, AC:B being an 8, and AC1 being a 7.