The only really great AC game was the first, which was an actual assassin simulator. The assassin tombs in 2 were great, but the rest ranged from mediocre to bullshit. I never played Brotherhood or Revelations because they were more of 2, which was bullshit because it was supposed to be a trilogy anyways and I wasn't particularly enthralled with Ezio as a character. Didn't play 3 for a bunch of reasons, ranging from the bullshit canadian vision of 1700s New England (which was 99% farms) to the absolutely ridiculous shit they did with the story. I literally said "they probably pulled some Deus Ex Machina bullshit with Desmond", and was so bothered by that possibility that I went and looked it up.
ACIV looks like a game worth playing, mostly because everything I've heard says that it's basically not an AC game in the first place, which sounds awesome.