I've been through a few of the main contenders but keep coming back to a Russian Big Muff - it's got the 'tone suck' when bypassed, but it's not a difficult mod to change it to true bypass. There's just something about the filthy, scooped sound of a Muff that does it for me.
The standard advice on here is to use a Boss LS2 (or similar) to add some clean tone to help retain low end. Dirty, filthy bass is really all about the mids, so once you've got that right then you've pretty much cracked it.
The DHA VT1EQ also does a broad spectrum of dirty bass...
http://youtu.be/cYpdDY3dMxo
the "fuzz" starts about 3'40" ish.