guess I'll weigh in with a couple of my choices
Some of the best rappers currently out there will fly under mainstream radio, but since this is a best rapper thread and I'll the names out there
Elzhi (a Detroit rapper currently with Slum Village, this guy can go toe to toe with Em when he was in his prime)
Phonte (this guy is 1/3rd of Little Brother, he definitely should get more attention for his skills)
Pharoah Monche (he's been around for awhile and technically he's a top 5 emcee)
Casual (he's a solo artist and also a member of Hieroglyphics, I think weak production are one of the things that holds him back. but the guy is superb, an emcee's emcee.)
Nas (I voted for Nas in this poll because he's been consistently on point lyrically for years now, his albums are fairly decent and he never fails to disappoint. He even influenced Eminem, Em's first album Infinite was his attempt at emulating Nas)
Eminem (at one point Em was the greatest rapper to ever touch the mike but for some reason he changed his rapping voice and watered down his songs. I still like him but he lost some serious points in an effort to maintain his platinum album status)
Mos Def (could have been another person to claim the greatest emcee title, but he's obviously focusing more of his time and energy to his acting career. His music has suffered because of his acting, but if he had released 2 or 3 more albums like his first one he would be the greatest)