Off the top of my head, in no particular order outside of the first couple
Nas, Kool G Rap, Ghostface Killah, Biggie, Scarface, Andre 3000, KRS-One, 2Pac, Ice Cube, Jay Z.
My criteria is pretty simple. Lyrics, flow, consistency (albums), storytelling, metaphors, longevity. My problem with Eminem is that his best work doesn't age well, he hasn't really grown much as a rapper, and he has been trash for a decade. He was a great rapper but his recent stuff is just bad, and the older stuff doesn't really hold up in hindsight. Obviously my list features other people who suck now (KRS-One) too.
I'm hesitant to put 2Pac in the top ten as well. While he's a personal favorite, as a rapper he doesn't really hold a candle to guys like Biggie, Nas, Jay, etc. Or as a lyricist. But as a songwriter he was pretty damn good, and had one of the best voices/delivery in rap.
My problem with Em stans is that it's obvious most don't listen to rap outside of Em and a few rappers connected to Dr Dre. So they sit around marveling at Eminem's amazing flow while ignoring the people he got it from (G Rap, Nas, AZ, etc). Basically Eminem fans tend to be music plebes who only listen to him. How anyone can defend his last few albums is beyond me.