I have used over a dozen different distortions on my various synths. the 303/X0XBOX is a bit tricky due to the filter and the sound of the unit which can be quite *sharp* in the high end if you use a pedal that doesn't compliment it.
The EH Big Muff Pi *is* a great pedal but imo is more suited for guitar than a X0XBOX. Any basic overdrive (as mentioned the MXR distortion + is good and , ironically, an overdrive more than a "distortion"), Ibanez Tube screamer, or even something like the Danelectro Daddy-o will probably give you very good results. For something a bit more aggressive my fav is the Boss FZ2. This has a great two band EQ and three different settings the last of which is more of a very powerful gain boost than a Fuzz and is *great* for some beefy sounds. YMMV