Maybe the best rapper of 2000 and later, but not of all time.
I liked Eminem, but as far as rap goes, it has been downhill since Tupac's death. Tupac's political/social rap rap (ie. Part Time Mutha, Changes, Ghetto Gospel, I don't give a ****, Words of Wisdom, etc...) are the best rap songs I have ever heard. 2Pacalypse Now, with that 80's sound (particularly heard in the song If My Hommies Call) is my favourite old school rap album; an era Eminem didn't touch (although he was the same age as Tupac, so it's not like he was too young). Tupac's battle raps also blew Eminem's out of the water, Eminem has nothing that comes close to Hit Em Up, not even his cover of Hit Em up (Quitter).
That all said, I liked Eminem, but he was a step down the hill from where Tupac reached. I don't think he would be nearly as popular if he wasn't white.
As for pop artists of this century (which is the actual thread topic), since we're counting sales; you should consider Lady Gaga in the mix; she has had a buzz that has eclipsed Eminem's, and far surpassed what he had at the beginning of the century. 80 million albums and singles sold in two years is nothing to scoff at; she has a huge buzz, and commands the respect of the old school artists (like Bowie, Deborah Harry from Blondie, Elton John, Bruce Springsteen, Madonna, etc...) that Eminem does not. I think she'll go further faster, especially if she continues evolving (her next album is a rock album) rather than sticking to one genre (Eminem has been on shock rap for a decade).
This all said, the 60's through the 90's were superior decades for music in my opinion. The past 10 years have felt like a weaker version of the 90's, without the same sort of diversity.
I describe myself as a little dose of toxic masculinity.
