The ones I know about:
Boss VF-1
Boss SE-70
Boss SE-50
- all discontinued...find on eBay
TC Electronics G-Major
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Alesis Quadraverb 2
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There's dozens more out there...and you can find most on the used market for really good prices.
These all vary in price and quality, and most do something better than the other, so it's hard to say what's 'Best'. Obviously, the higher the price, the better the quality (Eventide...G-Force), but for a good, solid, processor that will give you all the effect variation you could possibly want, I suggest the Boss VF-1 or SE-70. They're made for guitar, but do bass really well. They're also designed for live use, so switching is pretty quick and editing is fairly easy.
Hope this helps.