If your bass sounds good thru your current system, I'd go with a compressor. If you're going thru the system it makes the sound guys life far easier. And if you're live thru an amp, it will smooth out your sound and give you more control over your dynamics. Also a compressor in front of any other effects makes them sound better by giving them a cleaner more consistent signal. One other point don't try to
save money on effects. While there are some great bargains out there on a few types of pedals compressors are a pretty much your get what you pay for proposition. The ones to check out are the Mark Bass, Demeter, Kelsey, FEA, and the MXR M87.
This guy does great reviews here:
Compressor Reviews
As far as other pedals. it depends on what you want your bass to do. Some of the natural distortion pedals like the Sansamp, VT, and the Myco are great for getting more of a tube sound out of a real clean rig. If you play a lot of urban gospel an Envelop filter is cool, and an octave is nice to fill in the sound in a smaller group. A chorus or delay is great on a frettless. A lot of metal and heavier players like an over drive-distortion-fuzz. I'm personally not a fan of the multi effects, my experience is most cut corners or take away from the good tone of a good bass. And if you need a pedal to make your not so good bass sound good, you have problems an effects pedal can't solve. You want to start with a
good sound and add effects to enhance.