Indeed, a (green/Russian) Big Muff, but you also see tubescreamer-like pedals tuned for bass very often. Oh and the Darkglass pedals (B3K and VMT) seem to have become instant classics.
3 classic bass pedals: Russian big muff, bjfe blueberry, and mutron iii. Keep in mind that bass specific pedals hasn’t been a thing for a long time. It’s actually how I got into building pedals, I started nodding pedals for more bass, then building from the ground up.