I've played 1, 2, Brotherhood, Revelations, and Black Flag and the Aveline DLC.
I kind of liked AC 1, to be honest, even though I will freely admit it's a glorified tech demo with some very little content thrown in to mask it.
AC 2 was a...mixed bag. The gameplay was mostly the same with some additions which was OK, but the setting rubbed me the wrong way in a lot of cases. I still managed to finish the game but I liked 1 better.
Brotherhood and Revelations were just more of the same again with some additions. Having your own assassins was cool and I liked it but I don't think the changes in Revelations were really that interesting - I pretty much never used the bombs aside from the places I had to (because tutorials). The hook thingie I only used when climbing because it's pretty hard not to, but I don't think I used it in combat.
Black Flag I played just about a month ago and it was
awesome. After finishing all of AC 2 I wasn't exactly sure I wanted to do more assassining since I pretty much had my fill, but I heard Black Flag was good so I waited for quite a while before I played it. It blew me away - it was more open-world-y, I guess, and less stuffed in a city where you had to stab people. From premise alone, I wasn't sure I'd like it as much - AC but the main character is pirate? Meh, I thought. But now Edward Kenway is my favourite character in an AC game, as well.
The Aveline DLC...eh, it happened, I suppose. I would not suggest playing it, really - I only did it out of morbid curiosity, but it's not really that good. Really short and really straight forward - it's pretty much the exact opposite of the other games.
Chimpzy said:
I might give AC4 a try if I can pick it cheaply in a sale. From what I've heard, it seems to be the odd one in the series with its focus on sailing, swashbuckling and naval combat.
Do give it a go if you have the chance. As I said above, I found it really fun - the gameplay is different, for sure. Well, occasionally, you do go in a city and have to stalk people in the streets but most of the game is you sailing to various islands and doing other stuff.