The first thing I would advise is, whenever possible, buy bass-specific pedals. The biggest problem with using guitar pedals on bass is the massive cut in bass frequencies, and losing your low end is counterproductive. Bass specific pedals are made with the bass frequency range in mind so you keep the low end and get the effect as well. My second discovery is that having Wet/Dry controls is almost mandatory. Wet and Dry knobs control the volume of two different signals; the Wet being the effected signal, and the Dry being the clean signal. This way you can dial in as much or as little effect as you like while having your clean level unaffected.
My personal rig includes: the MXR Bass Fuzz Deluxe, MXR Bass Envelope Filter, and the EHX Freeze. Feel free to come in and check out these and many other bass specific pedals.