Deadline is reporting that Marvel is currently testing five stars for the role, including Joel Edgerton, Jack Huston, Jim Sturgess, Lee Pace and Eddie Redmayne. Garrett Hedlund, Sullivan Stapleton and James Marsden are also rumored to be in contention, so the "list of five" that have been confirmed may not be the final or complete list.
As Deadline points out, Marsden is an odd choice because he's already played Cyclops in the first three X-Men films, but those weren't Marvel Studios movies and aren't part of the Marvel Cinematic Universe; before taking on the role of Captain America, Chris Evans played Johnny Storm in Fantastic 4 and Fantastic 4: Rise of the Silver Surfer, so there's nothing to say Marvel wouldn't do the same again. A more substantial impediment to his appearing in Guardians of the Galaxy might be if he's back in the X-universe for X-Men: Days of Future Past, which yesterday confirmed that at least some members of the original X-Men cast would return to reprise their roles in the time-traveling epic.
Edgerton, Huston and Hedlund may have an edge in terms of landing a big tentpole franchise lead, since their names come up attached to nearly every big action film that needs a lead. Their broad appeal might help sell a film like Guardians of the Galaxy, which isn't as well-known as the other Marvel movie properties.