I did a lot of research into this recently. My personal feelings:
The Tone Hammer was good, but I thought the dirt got a little flatulent if you pushed at all.
The Hartke Bass Attack sounded good and had an incredible price, but it was large and I couldn't force myself to believe it had acceptable build quality for just a hundred bucks.
The Darkglass B7K was incredible, but the dirt is pretty aggressive and the price is in the stratosphere.
The SansAmp BDDI was pretty good at getting a subtle retro tone. I struggled with the EQ, though. Perhaps the ParaDriver would work.
I eventually got the MXR M80. The dirt is kind of aggressive and pretty modern sounding, but I can dig it. The Color switch can make getting the right tones out of the two channels a head-scratcher. But overall, considering the DI, the EQ, the dirt, the build quality, the size, the reliability... It really seemed like the best choice.
I have a big review and some sample sounds up around here somewhere, if you're curious.