The most musicaly influental country in the last 40 years.

[The Yosemite Bear]

Uh, fellas, are we possibly forgetting an entire genre here? Like it or not, hip hop is the biggest thing to happen in popular music since the Beatles, who, incidentally, did much of their influential work before the time period specified in the OP. I'm going to have to go with the US here.

also Muddy waters/howling wolf with the electric chicago blues. (ok that was 50 to 60 years ago when that scene was smoking hot, but everything since then including hip-hop owes a lot to wolf and waters)

[Rye]

Britain. The Beatles apparently had a big impact, obviously, helped shape Rock music as a whole and are renowned for innovation (though they're not my speciality, so I'll just accept that on face value).

Then of course, britain had Black Sabbath, the lords of all fuckin' metal. Diminished fifth use for the win! Metal, as I'm sure you all know, is my genre of choice, and the fact it was made here really seals the deal beyond all else to me.

Then there was punk, which, while I might not like it that much, was an important part of musical evolution that also gave rise to hardcore punk, which was integral in the evolution of thrash metal, which then obviously lead to the more extreme forms of metal. Carcass, from Liverpool, are also personal heroes of mine, not for their grindcore -related stuff, but for their fusion of deceptively simple hard rock styled groovy death metal, they founded melodic death and goregrind.

Grindcore also started here, from Birmingham. Black metal also started here, in Gateshead. There was all that NWOBHM stuff like Iron Maiden and Judas Priest etc. Then there was all that speed metal.

The beginnings of modern industrial and dance music came from here also, thinking of gary numan, new order, creamfields, et al, though dance as I understand it actually started in america, it was mainly popularised/innovated here and spread to Europe.