There have been numerous games released this year with good boss battles. I will add four of my own and then allow everyone else to suggest bosses to add to the list. I will count votes for various games and post the top ten at the end of the month (November 30th).
Mr Freeze - Batman Arkham City
Reason: Possibly the only fault in Batman Arkham City were the boss battles, with this being the only exception. Mr Freeze is unique because you cannot attack him in the same way twice and the only rule when fighting him is stay hidden. I like this boss fight because it is unique when compared with others in this game and Arkham Asylum.
The Lambent Leviathan - Gears of War 3
Reason: The Leviathan in Gears of War 2 was possibly the only descent boss in that game (also being one of the only ones) but the Lambent Leviathan looks almost unbeatable at first, but is defeated with only minor difficulty once you manage to obtain the Silverback battle suit in the second mission. The boss takes up the first fifth of the game and is only defeated when an army of creepers are dropped on it at the end of the sixth level of the game.
GLaDOS - Portal 2
Reason: I didn't find the boss itself to be very impressive but the fact that the producers managed to fit a boss into a puzzle game and that they even managed to revolve the entire game around it.
The Joker - Batman Arkham City
Reason: I know that I already mentioned a Batman: AC game above (Mr Freeze) but this 'boss' can't go without being mentioned. It is debatable whether this is actually a boss at all as Joker is only a minor part of the fight but it is worthy of being called a boss battle in my opinion. At first the boss seems far to easy, the Joker jumps down off of his platform and the player just beats him up, the only way that the Joker can hit the player is if the player stands completely still. But then, after the Joker is knocked down, he calls in about twenty of his goons, along with a one armed man holding a hammer and a goon infected with Titan. This is one of the only boss fights that I struggled with in Batman AC and it was a relief when I finally finished it, which is why it is worthy of appearing on this list.
These are only a few. Please suggest your own ideas onto the list. Just name the boss, game and reason.