I'd definitely say the Boss GT-10B. First, I've had one for years so I know it can deliver on tone and flexibility. Second, the GT-100 just came out, and while there isn't a bass version yet (and may not be for another year or two based on past trends), it's resulted in a nice price-drop on the GT-10. You should find one brand new within your price range as a result, but failing that you'd definitely be able to find a second-hand one in mint condition.
The Boss ME-50B is a great unit, very usable, sounds good, and extremely cheap due to its age, but I wouldn't get one because they're showing their age now in things such as poor tracking. You'd only need to spend a couple hundred bucks more to get the GT-10B, and it's worth it IMO. Rather than get a newer, smaller unit, I'd recommend you aim for something larger (and thus more flexible), but just a bit older so that you can take advantage of the price drop. I bought mine when they first came out and I should have waited; the price dropped 20% within three months.
I have no experience with the B3 but it's getting recommended like crazy here at the moment. My experience with older Zoom units has been less than positive, as I found their effects to be either too thin for bass or too unsubtle. I also found the interfaces, while fairly quick and easy to use, to be very cumbersome when it comes to editing effects and switching between them, which was annoying considering the factory presents were mostly unusable (as they always are with multi-FX units). Zoom may have lifted their game recently, but Roland's game never dropped so I've stuck with them.