The Legend of Zelda
I am a HUGE Zelda fan, but I do not see them being able to pull a Zelda movie off. I do think that it is possible to make a good Metroid movie.
Metroid would be easier to write and do well. I love Loz but a movie adaptation would have to scrap allot of key things that make the games good. Also, the LoZ series has a huge back-story that would be nearly impossible to incorporate into a movie. You know, with Link reincarnating as various heroes.