In order of priority:
1) Fantastic Four
2) X-Men
3) Spider-Man
4) Daredevil
5) Ghost Rider
6) Blade
While X-Men would be the biggest "get", the Fantastic Four bring far more to the table in terms of things they can add to the universe. I'm not entirely sure what they got sold off with, but at the very least we're talking Dr. Doom, Galactus, and the Silver Surfer. Those three, alone, make getting them back important. So in terms of adding to the universe that Marvel's trying to create in their movies, the Fantastic Four bring the most to the table.
After that, though, the X-Men would be major. They bring FAR more to the table than every single one of those other properties combined, but everything tied to them is mutant related, so it'd be in its own little ghetto separated from the rest of the Marvel Movie Universe. The only real crossover I can see there are Quicksilver, Scarlet Witch, and Magneto. I have to say, though. Fox scored a MAJOR coup when they locked up the entire X-Men franchise. Really, the X-Men by themselves are easily the equal of the rest of Marvel and DC's entire lineups, not only in terms of heroes but of villains as well. It really is a franchise with an inexaustible amount of material, and there's no way Fox is ever going to give it up. Nor will they have to. There's so much material there, so many different heroes and villains, that they can mine the X-Men until the end of time and still have plenty left over.
After those two, the rest are pretty unimportant. Spider-Man would of course be nice, but he doesn't really add anything to the Marvel Movie Universe aside from Spider-Man himself. So unless he becomes an Avenger, there's really no point to his presence. His villains certainly aren't necessary either. Green Goblin, Sandman, Dr. Octopus, etc, don't really cross over to other heroes and make appearances as Avengers villains, so they're all pretty unimportant all around. The same goes double for Daredevil, who is himself only a tenth as interesting as Spider-Man, and his villains are likewise only a tenth as interesting.
As for Ghost Rider and Daredevil, they'd be useful if Marvel expands its supernatural lineup. Seeing them co-exist with Dr. Strange would be fantastic. But if Marvel doesn't put out a supernatural lineup then their presence is pretty unimportant, as well.
