None are essential.
But...my top two:
But...my top two:
- Having a DI pedal (w/ some eq and maybe even overdrive options) may save your butt sometime: like when an amp crapped out on me at an event and I was able to run the bass through the DJ's set-up (DJ followed the band); in NYC I do not cart a back-up amp, but throwing my Tech 21 SABDDI in the gig bag has proved useful.
- Overdrive or fuzz, as others have said