There are a few game series that I will almost always keep buying as they release new games. The Assassin's Creed series is one of them. I almost didn't get the newly released Assassin's Creed IV: Black Flag today. Up until Monday, I had assumed that they were only releasing it on next-gen consoles, the Xbox One and the PS4. I'm not sure how that information stuck in my head. I guess I heard them say something in an early demo video and just assumed. I was a bit sad, since I'm not getting a next-gen system right away, but I got it, it's a business decision. Then I saw that it was being released today and I thought to myself, “The next-gens aren't even out, yet. How can they be releasing it?” Then I saw it was for all systems and when I looked into it, all reviews said both versions were the same except the next-gen ones looked a lot better. I can deal with that. Look what they did with GTA V? That game is gorgeous.
The Grand Theft Auto series is another one I will always buy. Those games are always so well done and polished, that I have no worries about quality and getting my money's worth. Though I may not like to admit it, but when I think about it, the Sims series is another one I will almost always buy. But, I don't buy all of the expansions as I usually don't get enough play time out of the $30 or so for them. If they'd charge $10-$15 for them, maybe.
I also don't really pre-order anything anymore, with the exception of GTA V, since I knew what I was getting with that. The pre-order that I did previous to that didn't live up to expectations (SimCity.) There really is no need to pre-order unless they have some crazy exclusive items or something, but you can usually still buy the DLC. Like AC4, I just went into Target and bought it. There is never a shortage anymore.
As for Assassin's Creed IV, I'm always a sucker for pirate-themed games. It's one of those times in history that I always romanticize about. It's one of the few along with the wild west (Red Dead Redemption) and the Mafia in the 1920s (no top game here, yet) that usually catch my attention and cause  me want learn more about it and the AC series definitely doesn't skimp on the historical information. From everything I've seen, this one looks to handle the life of piracy very well. Here's a nice “Assassin's Creed IV 101” video that Ubisoft put together. (Video will open in a lightbox, so you won't leave the site.)
Are there any game series you will always buy, or maybe some you will no longer buy?