I won't tell you which is the best becasue each has its pros and cons. I always used Rhino Mocks because it's free and there is a great community around and easy to find any help.
TypeMock is not free (I guess). However TypeMock is great if you want for example to test the legacy code with private or static types an members. Other mocking frameworks can't do it. A drawback is that it doesn't force you to write testable code which has other benefits that just the testability.
I don't know JustMock so can't tell you how it works.
I would advice you to try them all and to pick un one with which you feel comfortable. However if you need to test legacy code, chose than TypeMock.
I hope others will give you more details.
