Seventh Generation (2005-Present)
By the way, I agree with you on your issue with sequels. But the issue is that developers are reluctant to take a chance on a new IP in this climate and have it fail when they can simply continue a succeeding franchise and have guaranteed sales. It sucks, but it's true. Take DiCE's newest IP for example, Mirror's Edge. A first-person platformer that some either loved or hated. It was original. I really wanted to like it. It received mixed reviews for it's controls and bland graphics (white and orange, bleh!). But at least they tried and I really wanted to love it. The concept reminded me of the days of "Conc Jumping" in Team Fortress and perhaps if I played it on PC instead of XBox, I would have liked it more. But unfortunately we got the news this past week that the sequel will be no more.
I actually took sequels into account when taking my pick for 7th gen. LittleBigPlanet was my 2nd choice there and I agree it's a great fun game and probably deserved a top 3 spot. I don't dispute what got the top 3 spots, though. Those are terrific games. BioShock was obviously my first choice, so no arguments there. The story was fantastic. I hate that the movie was cancelled, but that may be for the best. I think the story would make for a great movie, but directors tend to always lose the story in game to movie translations and they turn to ####.
I picked Assassin's Creed 2 because I felt Assassin's Creed was the new IP from the era that sucked me in the most. I bought the first Assassin's Creed, and although I liked the game, it was disappointing in how short and repetitive it was. It also had no replay value, thus I traded it in a week later. When Assassin's Creed 2 came out, I was still very excited, but remembered how disappointed I was in the first, so instead of buying the game, I chose to rent. I loved it this time, as they fixed everything that annoyed me about the first and added so many new gameplay mechanics that I felt if I had bought the game, I would have had plenty of replay value. Of course, this past year Assassin's Creed: Brotherhood released and I chose to rent again. Not because I was disappointed in AC2, but because I feared it may be more of an expansion of 2. It was, but it was a huge expansion that I feel is worthy of owning. The added multiplayer is a bonus. You can find my review here on the forums.
Again, I agree with your distaste for sequels, but I wouldn't have felt right picking Assassin's Creed 1. Assassin's Creed 2 was the more worthy choice, plus Ezio was a more charming, memorable character than Altair. I also made the pick because I had made enough first-person shooter picks and was trying to stay away from those.
I think the rankings were good and fair. It's a hard category to rank. Nice job.
edit: I meant to mention that it seems that the only developers taking chances on original IPs and game ideas are indie games. There are a host of gems on XBox Live Arcade, Steam, and Playstation Network. While not all indie games are good games, it's a good idea to take a chance on them every now and then. Especially if you can get a chance to play the demo.