Generally speaking I hate this type of Question..because it often comes off as elitist, but I have never considered you to be one S>T<W, so...I give this one a go.
OK... I grew up on Thrash Metal, and Metallica brought me into Metal. So from that standpoint, I always have to defend American Metal. Plus... Death Metal was starter in the US, and two of the greatest Power Metal Bands are American (Iced Earth and Kamelot)..... Unfortunately..NuMetal is also a uniquely American contribution (if you want to call it that)
Conversely... In europe..Metal is almost Mainstream. It's everywhere, and becasue it infuses so much of the society (particularly the Scandinavian Countries) that the diversity and overall quality is simply a notch above America. Gothic and Symphonic Metal is barely heard of here in the US, where as, the genre boundaries seem musch less soild in Europe.
You have Gothic Metal bands like Lacuna Coil, that also put out "Pop" Metal songs like "Closer".. the Guitarist for Vader (Badass Death Metal) also put together the Pop Metal band UNSUN. Then you have the originators, like Black Sabbath, and the elder statesmen like Judas Priest and Iron Maiden.
So...on the whole..I would say Europe wins..because with so much quantity..you can't help but have great bands emerging.
BUT...that said..I think Band to Band..the best in America can stand toe to toe with the Best in Europe. It's just that Europe has SO MANY great bands.
One thing you are missing in this though is the explosion in the Middle Eastern, Asian and Latin American Metal scenes.
I have lately found simply BRUTAL bands form Israel, Jordan, India, etc......
As for my favorite European Bands......
Majest of Silence
Swallow the Sun
the list goes on and on........
(My personal top 10 bands)
Iron Maiden - England
Iced Earth - America
Bolt Thrower - England
Metallica - America
Nightwish - Finland
Sabaton - Sweden
Death - America
Eluveitie - Switzerland
Epica - Netherlands
Swallow the Sun - Finland
And thanks to which ever cocksucker gave me the TD......