Instrumentalist recommended chorus or an octave. I’ll add flanger as a possibility - yes, there is certainly some similarity to chorus like PhatBottomBass1 said, but there is a unique trippy feel/sound to it, and in particular, there isn’t that "sounds like you’re out of tune" aspect that chorus can cause with the bottom end.
Besides phaser, chorus and flanger, the other modulation effects that come to mind are tremolo (kinda cool imo, though not necessarily a standard bass effect), univibe and vibrato (definitely not a standard bass effect).
Delay is another possibility — for one, it can be used as a reverb-like effect without the "blurriness" an actual reverb will cause.
But really, unless you have a specific need for a particular song, you don’t really need any effect. How about a utility pedal/box like a DI and/or cab sim?