I like the Boss MT-2. I tried it out at guitar center with other metal pedals and I felt it got the best sound, I used it with an eq pedal that made it sound even more badass.
EHX Users Guild Member

B52 AT100 & AT412 Cab
Fender MIM Satin Strat
EHX Russian Big Muff
EHX Memory Man w/ Hazarai
EHX Holy Grail
Dan Electro EQ
Ibanez Weeping Demon Wah
dude watever u do dont get the metal zone, its the thinnest sounding piece of crap ever. get metal muff or i heard metal core is good, but metal muff should be ur best bet
buying a single pedal is not going to give you better tone, its based on a whole crapload of things. Amp, speakers, etc etc.
I've bought, sold, and traded more gear than I care to admit.