Besides Traveller- #1 on my list- check out Alternity: Star*Drive, Fading Suns & StarHERO for space opera. GURPS should handle the genre as well.
In addition, there are other good games that are less flexible- Gamma World, Space:1889, Cyberpunk, Paranoia, MechWarrior, Spaceship Zero and more.
And actually, the last 2 are actually kind of space opera, but very narrowly focused on a particular subset of the genre.