Your budget has a large impact on what can be seen as the "best" amp for heavy metal. Under $500: -Peavey 5150 combo (used) -Peavey Vypyr tube -Several Randall heads -Crate Blue Voodoo $500-1000:...
Honestly, the best one is the one you pick out. I personally like the HM-2 by Boss. The true best option in my opinion however is to buy a Mesa-Boogie amp. AMAZING overdrive
Once again there is no such thing as "best", as the guitar tone varies greatly on different variables and personal tastes. Higain amplifiers from manufacturers like Mesa, Peavey, Engl, Marshall,...