I agree with Eye Brand Man it depends on what time periods your speaking of.
If you're talking in the 1920s up to mid-1930s Ulster, Utica and Imperial all made well-built, solid knives, though I might give a slight edge to Ulster. Imperial started making shell handle knives in the late 1930s, while they did use quality steel knives, I would not say that shell handle knives compared to the quality of the earlier solid bolster Imperial's.
I don't know much about the early Colonials, but the later stuff I've seen made after World War II was not as good as the others, IMHO. I personally thought the shell handle Imperial's of the 1950s and 60s were better than the Colonials made in the same period.