I don't think I could do just one!
So this picture has a few of my favorites:
Front L-R: Big Briar (now Moog Music once again)
MF-101 Low-Pass Filter,
Z Vex Woolly Mammoth, (Roland EV-5 expression pedal to control frequency of the MF-101),
Fulltone Bass Drive (also CAJ tube compressor, which I like but wouldn't list in my all-time favs).
The MF-101 and Woolly Mammoth are a set - if considered separately then I don't think I would consider them favorites, if that makes sense. And the Fulltone is just such a great all-around bass distortion.
Oops, and of course the Space Echo in the back is up there, but it can't really be considered a pedal, now, can it?