are you using the pedal into a clean amp or to further overdrive an already crunchy tone (think Zakk Wylde)
seconded. also, which amp are you using? and which guitar?
EDIT: shitee. i should have read down.
Are you willing to boost the OD channel on the amp?
i mean, I'd say an OD would get you into megadeth territory with a classic 30, especially if you crank it. I'd even say the classic 30 could do 'deth without an OD if you don't crank it.
pantera would need a bit more work, though.
I'll second the big d, as ben said, it rules (i haven't tried the other ones he mentioned, i'm sure they're worth a look too!).
also, HBE have a more modern metal distortion pedal just out, it might be worth a look- but if you only want to boost the OD channel, i guess the big d (or even something like a powerscreamer) would be better.