Of course it depends on how much you are prepared to spend, but in the reasonable price range I would say that Beyer M88 is a really nice dynamic super-cardioid that would work fine for double bass. (A super-cardioid is what you want when you need the best possible front to back ratio, and a hyper-cardioid is what you want when you need to zoom in on something in a group.) A super-cardioid is supposed to be a little closer to the cardioid, and a hyper cardioid a little closer to a fig-8. But the terms are, it would seem, used completely at random so you will have to look at the polar plot to find out which is which.
If it is maximum separation you want, you may consider getting an omni and placing it ridiculously close instead (ie attached to the bass). This way you can get a really nice and dry bass sound, if you fiddle around a bit with the placement. It will probably be too dry for a jazz trio though...
