I'd go to a music store and try out different kinds of pedals. Then if I was still confused about what to get, I would come back and ask a more specific question. Something like "what is your favourite dirt pedal under $300 for bass?" is a lot easier to answer than "what is the single pedal I should get?"
What particular sound do you want that your rig can't do by itself?
I personally prefer single or dual effects pedals to multi-effect units, as I feel like a single well regarded effect is going to do that one effect better than a multi unit will do all of the effects. I'm also old and crabby and often prefer analog effects to digital ones (but the differences become less and less each year).
Here are some pedals I would look for to try out at my local or big box music store, in no particular order. Way Huge Pork and Pickle (bass od/fuzz). Way Huge Purple Platypus (octave up fuzz). DOD boneshaker (od/distortion pedal with a built in semi-parametric eq). Crowther Prunes and Custard (lovely madness). Morley dual boost distortion pedal (od/distortion + boost). EarthQuaker Devices Plumes (basically it's a bass friendly TubeScreamer type od pedal). Xotic ep booster is a fantastic sweetener without changing your tone and great as a solo volume bump (use the 2 internal dip switches and your ears to find the right levels for you). Darkglass Alpha-Omega ultimate (too rich for my blood). JHS Pandamonium (svt pre-amp and fuzz in one pedal).
Finally, search the used market in your area and the classified section here on TB. Best of luck to you in your tone quest!
What particular sound do you want that your rig can't do by itself?
I personally prefer single or dual effects pedals to multi-effect units, as I feel like a single well regarded effect is going to do that one effect better than a multi unit will do all of the effects. I'm also old and crabby and often prefer analog effects to digital ones (but the differences become less and less each year).
Here are some pedals I would look for to try out at my local or big box music store, in no particular order. Way Huge Pork and Pickle (bass od/fuzz). Way Huge Purple Platypus (octave up fuzz). DOD boneshaker (od/distortion pedal with a built in semi-parametric eq). Crowther Prunes and Custard (lovely madness). Morley dual boost distortion pedal (od/distortion + boost). EarthQuaker Devices Plumes (basically it's a bass friendly TubeScreamer type od pedal). Xotic ep booster is a fantastic sweetener without changing your tone and great as a solo volume bump (use the 2 internal dip switches and your ears to find the right levels for you). Darkglass Alpha-Omega ultimate (too rich for my blood). JHS Pandamonium (svt pre-amp and fuzz in one pedal).
Finally, search the used market in your area and the classified section here on TB. Best of luck to you in your tone quest!