Honnestly I don't believe there is any competition here. Urasawa's mangas are just made to become movies. I'm thinking of Monster obviously, but also 20th century boys (I think they made a movie out of it recently, but I heard that it was not great, I'd love to see this story in the hands of a big producer) or Pluton.I think a Monster movie is actually in the works, as a matter of fact. Or, at least it was - apparently it's being adapted into live-action TV instead: http://en.wikipedia.org/wiki/Monster_(manga)#Live-action_adaptations
But yesss! I totally agree. Monster's definitely a series that would translate well into film, and if they're going to do a TV series instead of a movie they'll probably have an easier time writing the script as well. I'd be sad if they cut any of the important bits out just to depict everything within a suitable timeframe, heheh. Monster is so rich and chock fulla goodness.
(20th Century Boys has already been adapted into a movie in Japan, by the way :o
http://www.imdb.com/title/tt1155705/ )
Steins;Gate would be great-- the story could probably be given great justice in the format length too.ZnK isn't done yet^
Also I would love to see One Piece fit in one movie lol
Gintama is episodic and rarely has arcs so can't be movie
Though Bleach and Death Note would be good, I also would like to see Stiens;Gate, Code Geass, and Guilty Crown made into movies would be amazing tbh