well, thinking about it i can advise so many pedals, its better to pick only one effect i find particularly tasteful on bass:
the MXR Phase 45
take it as a great way to introduce you to phased bass sounds... its diaphanous and ethereal, makes a great soloing support (if your in 'that' tripping state

) ...think of Pink Floyds Breathe, and you get some of it (not the same, but texture alike)