Weirdly, my Hardwire SC-2 Valve Distortion. I used it as a guitar overdrive originally, and when I moved to bass I brought it along. I have it set at very low gain and leave it on. It does a killer job fattening up my tone, and on nights where I'm having weird tone issues with the room, I often do better fixing it by adjusting the parameters on the SC-2 than on the amp! Not a common pedal for bass, but behind my Malekko compressor, which is also always on, the Hardwire is my MVP. 5 years, two instruments, still happy.
Playing bass in a band is my full time JOB! I'm blessed.