Definitely United States. And here are the two reasons behind that rationale.

#1 Genetics - No question here. What has Canada done lately for the gene pool. The tired, old, Champagne? Breeder Steve's Sweet Tooth and Shishkaberry are nice I suppose, but not cutting edge. Only thing the UK has contributed was The Cheese, which was a mutant Skunk plant, but nothing really new of late. Look on the major marijuana sites. All you see is OG Kush, Grape Ape, Granddaddy Purps, Purple Urkle, Skush, Chemdawg, etc. The US has the fire genetics at the moment, there's no denying that.

#2 Grow conditions - many will argue that you cannot beat the sun when it comes to growing. We can assume that each country should be able to create an ideal indoor environment, so if we took the best indoor from all 3 countries, you probably couldn't tell which one was the best. But if you bring some outdoor from the UK, some outdoor from Canada, some outdoor from California, and some outdoor from Hawaii, weed from the US will easily come out on top, Canada next, and UK last.