Bayonetta. The most fun I've ever had with a beat-em-up action game and one of very few recent games I've bothered to play to full completion. The plot isn't much to speak of, but the game makes up for it with excellent design and absurd humor. And fun. Insane, beautiful fun.
Though, it was released in January over here and in October 2009 in Japan, so if that doesn't count; Mass Effect 2.
Really, that'd be my GOTY if it hadn't introduced some less likeable [small]*cough*tsundere*cough*[/small] characters, taken away Shepard's sapphic space-pal-for-life [small](completely unnecessarily, the hell with the comic)[/small] and had a villain that was kind of a letdown [small](the Collectors, not the Reapers: they hardly even show up)[/small].
Of course, it's still one of the best goddamn games I've ever played, so there's that too.
And an honorable mention to Fallout: New Vegas for actually being a Fallout game. Definitely Obsidian's best game, they got the feel of the franchise completely right, so it's much more wry and dark, not to mention the fact that Obsidian have much, much better writers than Bethesda. If only they'd had a few more weeks to iron things out before release, it could've been so much better. The most irritating thing is to discover that you can't finish a certain quest without using the console.
It's never unfixable, but it really messes up the immersion when you have to dick around with the game's mechanics to proceed.