Source Audio Bass Envelope Filter Pro. It is ugly, it is digital, it is powerful. 6 presets are great for stage use. You can use an LFO to control the filter. It has 23 different types of filter, okay maybe 2 are really a phaser. A solid pedal indeed.
Ugly? I think mine is as sexy as hell. If I had to have just one efilter...then the BEF Pro is it.
If you like the Q-Tron and Bassballs and you must have only one pedal, why not the Q-Balls?
QTron and QBalls are pretty much the "same" sounding units....also, QBalls can't do the Bassballs thing. But the BEF Pro can.