Oxford and Cambridge serve their purposes quite well, but they've become too centralized. Imperial is good, too. I just don't think they really compare point to point to the Ivy League + MIT and Stanford. Harvard snatches. Yale snatches. Princeton snatches. Dartmouth snatches. Brown kinda snatches. Columbia snatches. Penn snatches. Cornell...
I'd say U.S., even though Oxford and Cambridge give Harvard and MIT a run for their money.