I guess I'll kick off the actual discussion since I have no idea WTF is going on here.
I think most people agree that AC2 is the best in the series followed by Brotherhood. Ezio is a well written, likable character, which is what really makes the games. It also addresses most of the game play mechanics that plagued the first game. So over all, it's a smoother experience. Story is solid as long as you ignore the present day BS they cram in there. If you're going to play any Assassin's Creed game, start with 2 and go from there. Revelations felt like a shorter ride and quite derivative though it's worth playing to end off the Ezio trilogy.
AC1 is probably the worst game in the series. I give them props on the art design and level of detail that went into the world. However, it just falls apart from there. It's a very tedious game that involves far too much repetition. What makes it worse is that EVERYONE wants to kill you, unless you put your hands together and pray. Literally. Combat is easy but not very fluid. The story and characters are wooden. Altair is just very wooden and unlikable in the first game.
AC3 is just okay. The story's not all that inspired and Connor falls flat as the main character. It's a very difficult game to get into. The first few hours really drag before you get to the meat of the story. I did enjoy the sea battles though. Glad it's going to be at the forefront of AC4. Who doesn't want to be a pirate?