(I haven't done one of these in FOREVER [mostly because they've been sucky for a while >.>'], so, why not? ^^)
I'm obviously biased, but, my pick for best game of 2010 would be...
Kingdom Hearts: Birth by Sleep.
Not only because it's part of my favorite fandom, mind you. Yes, Kingdom Hearts has done a lot of portable game system spin-offs, and today they'll be releasing yet another in the States, Kingdom Hearts Re:Coded. I know some fans have gotten sick to death of them and just want KH3 already. But, Birth by Sleep is easily the best of the portable KH game lineup. It's good enough to warrant a PS2 release, honestly.
(actually, I'd like to see that happen; more people need to play this game. D: )
Instead of introducing a not-exactly-intuitive playing system (KH: Chain of Memories), or making you do a sometimes repetitive series of missions in-between storyline events (KH: 358/2 Days), KH: Birth by Sleep is very much like a Kingdom Hearts platform game, like KH1 and KH2. It honestly feels like you're playing one of the platform games. There's a customizable list of commands (attacks, items, magic) like 358/2 Days, but you level-up in the traditional KH way, plus, through the different attacks you select, physical or elemental, you can enter a certain command style mode, that helps you deliver different, more powerful attacks against the Unversed. (Firestorm is awesome, and Ven's Wingblade is easily my favorite; it's TOO awesome. <3) Instead of summons, like in KH1 and KH2, you use dimension links, or D-links, where you connect to your friends and borrow their attacks and magic. Personally I like them better than KH1 and KH2's summons, which I rarely used.
The gameplay is just challenging enough, but it's also fun. There are a few occasions where I actually chose not to save the game because the boss fight or certain gameplay section was so fun to do. (Ven's encounter with Lucifer the cat in Cinderella's house, and Aqua's escape from Maleficent's castle being two of them) And being able to attack in the different ways provided by the command styles and D-links help make the job of Unversed extermination not so monotonous.
The graphics are of KH2-style quality, and text dialogue is at a bare minimum, so cutscenes are plentiful, but balanced. You visit Disney movie-themed worlds, and see their respective storylines play out, just like in KH1 and KH2. Except the worlds this time are mostly themed after older Disney movies, like Snow White, Cinderella, and Sleeping Beauty. The main exception is Olympus Coliseum, where you see a kid Hercules running around. (and also a kid Zack, which delighted the FF7: Crisis Core fangirl in me :])
And KH1 and KH2 fans alike will be delighted by the Radiant Garden world, where you get to meet some of the members of Organization XIII, before they lost their hearts, and listen to some pleasantly familiar theme music. I want to live there, honestly. :[
I was wary at first about the new characters, Terra especially, but they're all likable, even lovable, in their own ways. Ven, surprisingly, is noticeably different from his lookalike from KH2, Roxas. He's childlike, innocent, and kind. He's a bit like Roxas and Sora combined, Sora's occasional cockiness not included in the equation, thankfully. -.- Terra is strong yet flawed, mature yet naive, determined, but doesn't forget what really matters; his friends. Aqua, I'm just getting to know (I'm currently in Enchanted Dominion with her in the game), but so far she appears to me to be strong-willed, kind, protective, and optimistic. And she loves Terra about as much as I do, sharing the same optimism for him that I did, so that makes her a winner in my book. xD Plus, so far I already consider her to be the best female character the Kingdom Hearts series has got.
The voice acting is near-perfect, with Jesse McCartney somehow managing to sound similar but different from Roxas (he's getting better at voice acting all the time :]); Jason Dohring putting in a good, emotional (in the later parts, at least) performance, for this being his first voice acting role; Mark Hamill putting in a surprisingly believable performance as the trio's mentor Master Eraqus; and the man who helped found one of the biggest fandoms of all, Leonard Nimoy, putting in a fitting, dastardly and delightfully evil performance as Master Xehanort. And the person who performed the voice of Vanitas put in a shockingly good performance.
(Don't let ANYONE tell you who his voice actor is, or what he looks like under the mask. Trust me on this.)
The storyline is compelling and emotionally involving, with its own twists and turns here and there as well. It reveals quite a few details that are relevant to the KH series storyline both now and going forward.
I didn't play many new games in 2010, and I know that action-adventure RPGs may not be everyone's favorite type of game, but when it comes to this genre, this developer, and this past year, it didn't get any better than Kingdom Hearts: Birth by Sleep. And that's a good thing.
Kingdom Hearts: Birth by Sleep for the Playstation Portable: I highly recommend it.