Bryan Singer was very
involved with X-Men: First Class
He initially went to the studio and pitched the very idea for the project. At the time, Josh Stewart was developing the movie as a direct adaptation of the comic-book, but Singer's approach was a direct prequel to his movies. As a matter of fact, his treatment basically provided to be the backbone and foundation for the rest of the story.
It was Singer who suggested Matthew Vaughn to replace him when he had to drop out as director due to a prior commitment. He was still a very active producer throughout the entire production, all the way from pre-production until post. If you watch the behind-the-scenes footage on the DVD, you can see Singer was present on set for much of principal photography. He did interviews for the press on behalf of the film. First Class
was very much Singer's baby, and probably a big reason why it was the best X-Men film since X2