Hey I own LittleBigPlanet, Metal Gear Solid 4 and MAG. Those three games are amazing. Don't listen to that guy who told you about MAG being a Battlefield/MW2 hybrid, he has never played the game and it's obvious to those who played MAG, like me. Mag is more of a simulation than both of these games. It's not a arcade shooter like Modern Warfare 2, and it's not complete chaos like in the BF games... Although I do like both of these titles, don't get me wrong, I'm no MAG fanboy. I have rented MW2 and Uncharted 2 for a few days and both games look amazing ( graphics and gameplay ). I have only played the Heavy Rain demo but I'm pretty sure of getting the game one day or another... Here's what I think of the games in your list, they're all great games by the way:
Uncharted 2: Very colourful and flashy graphics but it is still an adult game with blood and weapons. There is a multiplayer mode, but it's nothing different from the other multiplayer shooters. The online modes are not very original and so are the weapons... To me the best 3rd person shooter experience is still Metal Gear Solid 4 ( I'll talk abou it later ). The offline, solo mode is lots of fun, but of course, after you have made it through two or three times, you won't find it amusing to do it again and again afterwards. It's a very good game however, with amazing controls and good physics. I would give it a 9/10 note because to me the first Uncharted was groundbreaking and this one is simply adding more to it. If you like the Indiana Jones movies but thought Indy wasn't badass enough, Uncharted 2 is your game, heh.
MW2: It's a very action-packed game. Everything in this game is short and sweet. There are a lot of ways to kill an enemy. The solo/coop missions are fun and different from one to another and they can be quite a challenge. The online experience is somewhat blurry to me. It's both lots of fun and yet it is deceiving. There's a lot of sniping going on, and you will most likely be insulted by other players who have a headset, but I must admit that it's lots of fun anyways. I've had lag with MW2 when I played it. I'm unsure if there is still lag with the game nowadays. The number of maps is currently lacking, but I'm sure they will add some more in the upcoming months. The graphics are stunning. It's an arcade shooter. You can kill people easily even if you aim from very far away or if you hit them in non-vital parts. It's kinda similar to the Unreal Tournament games, where you had an announcer and other over-the-top things going on. I would give it a 8/10 because I feel like they are simply milking the cow with this game and to be honest I prefered some of the earlier titles the Call of Duty franchise gave me ( the first one was a work of art in my opinion, this one is like bubblegum, you love it at first but you lose the tasty juice quite quickly. Of course there is always an addicting part to it since you can always upgrade your character, but most shooters do that nowadays. It's a very good game, but I feel like it is highly overrated. I also found the " No Russian " mission particularly sickening. This game certainly glorifies violence, perhaps even more than the GTA games.
LittleBigPlanet is probably my favorite in your list. I bought it when it came out and I still play it nowadays. This game's biggest attribute is it's level creation mode. It's an easy game, yet it can be challenging at times. If you have artistic skills already, if you draw, paint, do photography or build stuff at home, then you will love it. If you are only there to play levels, you might not enjoy it as much as others do, but still, the level list is growing and growing... Over 2 million levels online right now... Of course if you do not have internet on your PS3, you will find LBP to be very boring. You might experience lag if you play with people from faraway countries ( if you are in North America I don't recommend playing with people from Japan, Germany, England and other countries like that ). The game has DLCs practically monthly, sometimes weekly and the makers of the game have promised a lot of upcoming stuff. Through the months, people have found many ways to make original levels ( Pinball, survivals, ice hockey, basketball, Simon says, etc )... To make it short, LBP is to me the best PS3 game out there, a definite 10/10. It's a game for everyone, from adults to children, and the people you will meet online will be friendly and helpfull. You will never find somebody swearing or insulting you while you play because the game is just so positive in everything that it will make anybody who plays it feel good inside.
Edit: I reached the character limit! Well I just wanted to give MGS4 9.5/10 and MAG 8.7/10!