You need to know your vocal before you know what mic to use. And we need more information than someone will be rapping.
just a tip... if you put 2 seconds into your question you're going to get a 2 second answer. My 2 second answer is a rode nt2. Probably not for the best mic for you, but how would I know that? Im guessing what you need. It works for rap vocals. OR maybe your rapper needs a dynamic..we dont know yet.
Whats their range, etc..