Harry Potter will beat them out easily. The series has only been getting bigger since the second, plus it will also have the extra 3D boost.
The people saying Cars 2 make me laugh, the first did around 450 million and it's one of the most poorly received Pixar movies. Toy Story is a long lasting series with a rich history that had the full brute force of Disneys marketing power and it was critically acclaimed, Cars had none of these things and I'm betting it'll get the respectable Pixar 500 million at the box and it will teeter off into the wind. Kung Fu Panda 2 has an even better chance of outgrossing Cars 2, the first earned much more and was far better received.
Transformers 3 has a better chance, but based on ancedotal evidence, a lot of the 'normal' people I know didn't like the movie at all and I feel like they may skip out on 3 so that they don't get burned again. If anything, it'll maintain it's 800 million average, but I doubt it will go up by another 150 million to beat the new Harry Potter.
Mission Impossible has no chance. None of them have gone above 600 million and it's been in heavy decline. It also doesn't help that Tom Cruise isn't the critical or box office darling he once was.
Pirates of the Carribbean is such a hard one to predict. The last movie saw a decline in both gross and reception and even the common folk that don't rant on message boards were kind of down on the third one. Though, people do love their adventure epics and they definently still love Johnny Depp and the rest of the cast, so it's hard to judge it's future. I'm guessing it will do really well, but will suffer from a slight decline that'll give the edge to Harry Potter.
The other movies that'll come out and might get big numbers:
Thor: The casting and trailers looks good, but it's the first in the series, so I'm guessing that it'll get realy respectable numbers and then maybe a better performing sequel down the line. It also might be a miss, only because the character isn't as popular as your Batmans, Spidermans or Iron Mans.
The Green Lantern: It will crash and burn and all who see it's demise will relish in the fact that wearing ugly CGI clothing can be punishable by public shaming and box office bombing. It'll probably make it's money back, but I'm seeing The Incredible Hulk and not an Iron Man.
Sucker Punch: Looks awesome, but it's far to niche in it's execution to appeal to the general people and so it'll probably do a little better than Watchmen, but I doubt it'll get close to 300.
Captain America: If it's good, I can see it doing well, but as with most superhero movies, they tend to start of good and then really kick it off in the sequel and I can see the same happening for this and Thor.
X-Men: First Class: I have no idea what's going to happen with this. It's most likely going to be a fantastic movie and the X-Men brand is still going strong, but I can easily seeing the fans rushing to the theatres, while the general public ignore it as a fan fiction piece. Look's cool though.
As with every year, we might even get a huge surprise and see an unknown movie hit it big. Battle: Los Angeles, The Hangover 2, Cowboys vs Aliens (OMG!!!!!!!!!!!), The Smurfs (Oh NOOOO!) or Super 8. Hell maybe District 9 - 2 will come out and conquer the world, I wouldn't mind.
Bet with Conegamer and AussieGecko that the PS3 will have more exclusives in 2011 than the Wii or 360... or something.