1. The X-Men
The X-Men is a classic set-up where the team is focused on finding and protecting mutants from the rest of humanity, in a nutshell. The fact that they're also trying to remain hidden ups the stakes and makes it more interesting than just a monster-of-the-week in a find out what's going on with this new mutant that has shown up in some form or another. Though it's already better than monster-of-the-week though because the premise is usually such that the people the team is tracking and going after is already sympathetic to them and they're trying to help rather than harm (even if it doesn't always work out that way).
2. X-Factor or Heroes for Hire
3. New Mutants
The New Mutants is a fun premise in that they're kind of the B-team to the X-Men, in a way. I like that they're kind of in the shadow of the X-Men sometimes, and that they're young and are still trying to get a handle on everything - even more so than the X-Men. In the beginning they weren't even being groomed to become an X-team (or at least, that's how Prof X rationalized it anyway before soon realizing that yeah, they're basically going to have to be groomed to become an X-team) and were focused more on learning and going to school and such, just crazy stuff kept happening to them. The reason I put them as distinct from the next title is because I like the idea of the team working and living in the shadow of the bigger and more famous team who are veterans and know their stuff. When we playtested Worlds in Peril we had a lot of fun running games where the A-list super team got taken out and the B and C strings were called up for action out of the blue and it was great, great fun.
4. Wolverine and the X-Men
This book concentrates on the inner-workings of the school and being a student that attends it. It's got high school drama and has the benefit of being able to focus on the stuff going outside the school as well as inside. Stuff happens when they go on field trips to outer space, the savage land; when teenagers get angsty and have omega-level powers; when enemies of the X-Men come to visit. It's a really awesome premise for a comic, and a great one for a game as well.
5. All-New X-Factor
The more recent X-Factor book had the team being sponsored and brought together by Serval Industries under a big-wig CEO with a ton of cash, claiming to only do so for benevolent reasons. Working for a corporation with complex motivations is a neat premise - it's too bad the comic got cancelled, but there was a lot of interesting stuff going on in it. A private jet and everyone living together has a familiar team premise, but they're living in a corporate HQ, doing stuff that must ultimately service the company's interests, presumably, and access to resources to deal with stuff all over the world.
6. The Fantastic Four
7. Uncanny X-Force
8. Cable and X-Force
While the Cable and X-Force premise would take more doing to set-up I think it could be a lot of fun. The book revolves around Cable's abilities - particularly his precognition, which gets augmented and goes out of control in the book. Whichever way you choose to set it up, the premise is that the team becomes aware of dangerous events that take place in the future X amount of time before they go down. It's their job to make sure those events don't take place.
9. The Captain Britain Corps
The Captain Britain Corps are a group of soldiers from across the multiverse that specializes in taking down threats to certain worlds or to the multiverse itself. This allows for all kinds of play that could be as gonzo or as serious as the table wanted. Worlds where everything is the same except all the Avengers have bears, dystopic worlds that resulted in the deaths in certain key figures, or unknowable horrors consuming the fabric of reality or multiversal tissue itself. All kinds of possibilities exist when you can travel to any universe and planet, throw in time travel and the mind boggles.
10. The Avengers
The Avengers strategy is the easiest one to roll with and the one that I use the most often. Basically, each hero has their own personal life and does their own thing, but they come together when they need help or something big shows up. They've got resources if they need them, they've got other people they can call on, and when the dust settles they can go home and try to clean up the destruction their superheroing wreaks on their personal lives.