There is perhaps no more popular studio-quality microphone in the world than the Shure SM57. From vocals, to guitar amps to bass drums, the Shure SM57 is a workhorse that is lauded for its ability to handle a wide range of frequencies. While it may not be the best microphone at each application, it is good enough at each application to make it indispensable. For rappers, this is ideal. The range of vocal approaches from rapper to rapper will also vary. Having a good all-purpose microphone, then, is a sound investment. The Shure SM57 is also one of the most affordable professional-grade microphones, usually retailing for less than $100.
Sennheiser offers a full line of studio-quality products including headphones and microphones. The MD 431 II, for example, is a microphone geared specifically for vocals. By offering several noise-cancelling features, this microphone lets only the vocals come through and eliminates background noise. With a very high threshold before feedback, rappers can get loud if they want to without worrying about distortion in the recording. These two features make the Sennheiser MD431 II an ideal microphone for rapping in the studio or on stage.
While some microphones, like the aforementioned Shure SM57, are designed to perform well in a variety of scenarios, others are designed to excel in one very well-defined environment. Such is the case with the Electro-Voice N/D767a. This microphone is designed to excel in live vocal situations. Unlike a studio environment where sound booths are pin drop quiet, live settings can be noisy with multiple instruments bleeding into each other's microphones. The Electro-Voice N/D767a has been the vocal microphone of choice from artists as diverse as John Mellencamp to the scores of rock and punk bands on the Van's Warped tour. If you are mainly rapping in a live environment, this could be the ideal microphone.
For rappers in particular, the AKG D5 is a sound choice. In addition to being a microphone designed specifically for vocal use, its variable thickness diaphragm also makes it popular for public speaking. Because rapping can often embody both singing and a more speech-like delivery, the AKG D5 is more suited to rappers than vocalist in other genres such as pop, rock or country.