line6 spider III
nah, but srsly. 90% of a good metal tone is the cab and mic placement. i think the keyword is 'mellow', because harsh guitars destroy everything. celestion V30's, a good dynamic mic pointed well enough away from the centre. from there it's just preference.
i mean, really. logically, it'd be best to get as much distorsion out of the amp as possible, since tube distorsion is preffered over the analog clipping of whatever pedal you might have. people always go for EMGs since they're active and have a higher output. the only thing they're doing is driving your amp a tiny bit harder, making the tubes distort more. same goes for boost pedals, overdrives etc. they just make your tubes work harder. 808's add a little bit of crunch, sure, but it's the fact that they raise your input signal that matters.
engl, krank, marshall, diezel, laboga, peavey, mesa, can'tthinkofanymorebrands, whatever, they'll all work just fine. some heads are a little harder to dial in than others. the 5150 is, like previous comments have stated, idiot proof. even the rythm channel sounds fucking metal. go eddie! the only knob i've found to be a problem with that amp is the midrange knob. the more mids you dial in the sweeter it sounds, but it can fuck with other elements in your mix, so be gentle.
as far as recording equipment goes, i've heard a lot of people saying you need a 'warm', 'tubey', 'organic' (bullshit word of the century) preamp to go along with whatever mic you're using to record your guitars. i find that hard to believe since what you're essentially recording is a large preamp. all the crispy, warm good stuff is going to come from there, and if it doesn't you'd better start searching for another amp. the most justice you can do your amp is to have a clean a signal chain as possible, a 57 into a transparent pre with a lot of headroom will do.
conclusion: tube amplifier, v30's, microphone not dead center, raise input signal to drive tubes harder (boost pedals, active pickups etc), clean preamp with enough headroom = you're golden.
some people may have different opinions of course, but this is what I have found during my years of doing this professionally.