Barber Electronics Direct Drive.. It's more for of an overdrive pedal so you'll never get to the death metal stage but with some hot pickups, you can push it quite far.. As a blues rock overdrive, though, I can't say I've come across a better one.. It responds well to your picking dynamics and lets the tone and character of your guitar come through..
I've got the Boss Metal Zone as well as the DS-1.. I've never been to fond of the DS-1 but I've had some good sounds from the Metal Zone.. It does have a slightly digital nuance, though, so I tend not to use it if I'm going for a more vintage sound..
I'm also using the Marshall Jackhammer.. Must be said, though, that the distortion channel is crap.. Waaay too bassy for my ears, even after turning the bass down.. The OD channel, on the other hand, is a lot more usable and heavy enough for metal.. It also sounds warmer and less digital that the Metal Zone.. On top of that you get a great deal of control in terms of EQ once you get your head around all the knobs..
If I had to pick just one, though, I'd go for the Direct Drive..