The choice between #2 and #3 was probably the toughest decision to make on the entire list, so on another day this one might reverse. As it is, though, I'm going with what I feel right now.
1. Weedeater (stoner/doom/sludge)
2. Saint Vitus (traditional doom)
3. Metallica (thrash metal)
4. Incantation (death metal)
5. Autopsy (death metal)
In terms of depth of the musical scene it is of course nearly impossible to top the United States. Not only have I spent my life in the midst of it and thus been exposed more extensively to American bands, the country is just flat-out bigger than any other nation with a significant metal scene. It would be beyond fruitless to try tackling all of American metal in a single post, so I won't even attempt that. What I will say is that for me personally it is the great and long-standing American death metal scenes of Florida and NYC that are really the highlight of our national contributions to metal.