i've tried the following distortion types (as far as metal is concerned, so no DS1 or Tubescreamers here): MT2, Marshall distortion, modded and unmodded HM2, DOD Death Metal, peavey distortion, japanese amps distortion, Boss Hyper Metal, X Distortion, Boss Hyper Fuzz ...
the ones that stand out are:
1.) MT2 = if you are gonna use this unmodded, better get a Graphic Equalizer since the mids of this thing are too nasal sounding, i suggest you mod this into the bakemono (eherm). the disadvantages i see are: generic sounding because everyone uses it (but then again, metal is about riffs), expensive plus you have to either mod it or get an EQ pedal and in some cases, a noisegate. and just to add, another disadvantage with this (and with all high gain pedals) is that it kills your dynamics due to compression ... but then again, dont worry too much since metal is all about crushing power over dynamics. i just said this in case you decide to do blues stuff somewhere along the line...
2.) Boston Heavy Metal = now the stock version of this is a lot better than Boss MT2. you can play this baby without an EQ if you want - it is that decent already! ... you have the option to mod it, and it is dead cheap... i dont see why any guitar player on a budget shouldnt get this. if you are not on a budget, get a marshall halfstack or a mesa boogie triple rectifier... that will end the distortion debate.
3.) DOD Death Metal = one of the best chugging bass and mids ever! it beats the MT2 and HM as far as chugging is concerned! if you play a lot of groovy palm muted open chords, the Death Metal can make those heads bang. the only reason why i hate this is that since it doesnt have a gain control, the feedback shrieks at you everytime your strings stop vibrating. there is no way to shut the feedback off this aside from turning it off completely (which i dont recommend since DOD foot swtiches are flimsy), or getting a noise gate. on a decent amp, i doubt that you would crank anything past 12 oclock here... this thing is damn powerful! either get a noisegate or have it modded to have a gain control OR have your pickups shielded and potted perfectly to unleash the metal in this pedal.