I don't know man. Cortana was full of tight corridors where you would get sniped by those horrible Flood snipers. It was winding and narrow and just fucking ugly.
CE's Library was wide and tall so you could maneuver, backpedal, jump around, throw grenades, use rockets, etc. I wouldn't say that it was a beautiful level or anything like that, but it was designed properly for what it was: a throw back to doom-type levels where it's just you, your shotgun and the horders of hell. And the only way out is through.
Playing Library co-op with a friend is fun as hell. Playing Cortana is a pain in the ass no matter what.
That's just my opinion of course.
I agree that the levels where there are three-way battles are the best in the series. I don't know if we got much of those after Halo 3 though. Hell, I don't remember Halo 2 or 3 having anything as cool as that huge ice field at the end of Two Betrayals: Covenant wraiths, Hunters, Elites, Jackals, Grunts, rocket Flood, Flood carriers, etc. And you have to get through to get the Banshees. Badass.