Not underground in the UK, but doubt you will have heard this if in the US. Quite a few top UK grime artists.
Homebrew, they also have another group called @peace They are in New Zealand. it sounds different to most rap, due to the guys accent, I gues you could say its close to a english (british) sounding rap but its noit quite that, it is something completley different. they are down to earth and been makinga few albums in the last few years, although lately they havent droped anything lately so im not sure if they still makin music or not?
Sean Price, how the fuck this dude wasn't on the top of the rap game is beyond me. I havent heard a rhyme from him that wasn't slick.
Idk if they're exactly underground, but Slaughterhouse is dope as fuck and not many people I know of listen to them
how was it? nice list but how is hopsin underground? you listen to logics new album yet? i'm halfway through and am not disappointed
Most of the people listed aren't that underground at all, pretty well known people. But for my favorites I'd have to say Qwel from the group Typical Cats, Aesop Rock (not ASAP Rocky, this dude has the biggest vocabulary in music and I'd say is the best to ever do it), Milo, MF Doom.