Liked AC1, loved AC2, but my favourite game in the series was actually Brotherhood. AC2 had some great music and some really bizarre music and since AC is so reliant on atmosphere this did bother me. Also the plot twist 2/3rds into the game was Bioshock-levels of stupid for me and ruined the entire ending of the game. Brotherhood was more... it felt more coherent, more immersive. I liked the singleplayer part of it a lot more than I expected (besides the first hour, of course), and I even liked all the "present-day"-stuff in it for once. Actual effort was put into it. Also: The MP. I spent 3 weeks grinding the MP, I could not believe how much fun I was having.

For me, the series broke down at Relevations. Yes, they replaced Jesper Kyd (the man behind the soundtracks), but everything else about that game was just worse than the previous ones. I even kind of liked the story's potential, but it was handled so poorly on every level. Did they hire Zack Snyder or George Lucas and forget to put them in the credits list? Why is there a tower defense mode? What purpose does the little hook-thingy do, other than to very mildly change climbing to be even easier and faster? Gameplay, story, atmosphere - it all went down the gutter. Now you just chug a poison grenade at guards and kill an army at a time - and I thought the Arrowstorm from Brotherhood was broken! At least Brotherhood has some genuinely tough missions that involved sneaking and timing.

AC3 was pathetic.

I haven't played the pirate one yet.