Totally depends on your amp and the sounds you're going for. I like compression, some kind of drive, and delay. I also like a noise gate, EQ, and synth...
If you're happy with your base tone and just want some flavor, a drive with a clean blend is a great place to start. I like MXR distortions on bass but that's not a popular opinion. If you want good EQ, something with parametric mid bands is a great choice - I use a Wampler EQuator but I would love to have a QStrip or a Source Audio Programmable EQ2. The compressor I use is an Emma Transmorgrifier, but there are tons of good choices. My gate is a trusty old Boss NS2. Delay is whatever you like. My synth of choice is an EHX Hog and I think I'd recommend something from the Hog/Pog family for good tracking and fun sounds.
Another path would be to get an HX One as a catch-all. It can be any effect you want. Line 6 stuff sounds great on bass.