Assassin's Creed: Good idea, not very well executed
Assassin's Creed 2: Ironed out all the flaws of the first, more varied locations, interesting story, rewarding side missions/exploration/puzzles etc - Subject 16 stuff really stood out too
Assassin's Creed Brotherhood: Pretty much as above though much more refined combat - bit more confined location wise but still iterates and improves - Subject 16/Desmond stuff was really awesome
Assassin's Creed Revelations: Felt more like gigantic DLC than anything else, not really much new, not really interesting, plot was meh, it's contribition to the series storyline was quite minor and the Desmond related stuff was terrible - game as a whole was also very short in comparison to 2 and Brotherhood

Honestly I tell anyone who's interested in trying AC to skip the 1st one (read up on it if you want to understand some of the references) and start with AC2 and ACB - skip Revelations (again read up on it)