What makes rap good?

[bruin]

People here (and most people in general) always consider a few rappers to be in the elite: Tupac, Eminem, Biggie, Jay-Z, Nas, Rakim, Talib Kweli etc etc etc...

My question is what makes these guys good? What sets them apart from lousy rap?

Obviously lyrical prowess and cleverness is a factor, but what else? Clearly, speed isn't a factor when you look at guys like HOVA or BIG. I know people talk about the "flow" a lot, but I'm not sure what constitutes "good flow."

I kind of have my own idea of what makes rap good but I'd like to hear from OOT on this topic.