I would go with England as well.
The prog rock era started and peaked in England - Genesis, Gentle Giant, Pink Floyd, King Crimson, Yes, Jethro Tull...
Not to mention The Beatles, the Who, the Animals, the Kinks - all the major forefathers and members of the great golden age of rock.
But then again rock is American made, and if it wasn't for American acts like Elvis and his blues-based forefathers, these bands probably would never have existed, or at least not in the format that they did.
Not to mention the US has also had some great bands: Allman Brothers, Grateful Dead, Crosby Stills & Nash (although it's part Canadian too). And they spawned the grunge era, which I personally like a lot, especially Soundgarden. And then there are other American groups that have pushed rock to a different level, like Primus and Phish.
So it's tricky, but I would give the cup to Britain because of my affinity for the progressive rock era.