Oh yah personally I see rapping as a form of instrument in hip hop music. As a rapper basically you are constructing the variation and enhancing the rythym by layering it(on the beat) with the flow/skeleton of your rap.
Thats one aspect that signifies the skill of a rapper. And an
extremely important one.
The best beats are the beats that sound the worst. The better a beat sounds, the less room for a rapper to shine. Get a highly repetitive and rather boring beat and spit on it and keep flipping your flow (rhyme schemes, speed, style)
this is an excellent one for it
http://www.youtube.com/watch?v=J1D2pGn_-mkthere are different guys rapping on this beat, but they can flip their flows without spoiling the beat, its pretty constricted because hip hop instrumentation is different from other genres in terms of its terrible lack of variation but yeah, staying on the beat and maintaining flow is the key
http://www.youtube.com/watch?v=AqxBDN-cv08