AC1 - When I first played it, I thought it was kinda' repetitive, and it got a bit boring at times, but overall, I thought it was a good game, but after playing all of the other games, I didn't enjoy it at all.
AC2 - My favourite story of the lot, the whole revenge thing just felt epic, with Ezio hunting down a string of Templars who betrayed his family, over many years, and I loved watching Ezio go from noble Florentine teenager, with too much money and time on his hands, to master Assassin. It was the only game in the series that actually felt like a sequel. I also loved exploring all of the cities (Mostly Venice and Florence)
ACB - In terms of gameplay, it was better than AC2, but it felt too easy because you could just call a group of Assassins to kill someone/ a group, The combat felt more polished too, but yet again, it just felt too easy with the addition of the "Killing Spree" system, so you could just carve through a group of 20 guards in a about 10 seconds. Rome also felt a little boring, because it was the same scenery and architecture throughout. It did have multiplayer though, and while that was original and extremely enjoyable, nobody plays it anymore, so now you're stuck with a short campaign, and a pointless multiplayer. In terms of story however, it is completely forgettable, all I can remember is trying to stop Cesere Borgia for some reason, and I cant seem to remember why.
ACR - In terms of gameplay, it is the best, because everything just felt polished, but it too felt too easy (see ACB, because most of the additions in that, are still in this), it also jumped onto the bandwagon of "Slow-mo makes things cool!". Most of the additions felt pointless anyway, the bombs were useless apart from the killing bombs, the hookblade just let you climb faster, and seemed stupidly unrealistic, most of the time I think Ubisoft just want you to finish their game faster because it sure does seem that way. For multiplayer, it just seems exactly the same as it did in Brotherhood, but its better in everyway, yet it doesnt feel as good in Revelations for whatever reason. For story it seems similar to Brotherhood, a short story, and I don't see the relevance of it, you're trying to get keys to a library, and the templars want it for some reason.
EDIT - This applies to all of the games:
Desmond is a horrible character, in all of the games
--Joe
TL;DR - AC2 was my favourite.