In terms of rapping as a poetic/lyrical skill, Eminem. But I much prefer Kanye's music, both for the sound of it and what he writes/talks about. The samples, the singers' parts, the overall sound, the beats, and then the lyrical content I much prefer what Kanye talks about.
For me Eminem is much more hit and miss and controversial. Eminem I sometimes get the feeling goes out of his way to piss people off with what he says in his songs. Also I didnt think Love the Way You Lie was that great, it was carried by Rihanna, and The Monster was a bit of a pale imitation of that. For me, Kanye's material has been more consistent and there's been tracks of his I've liked on every album. My standouts are probably All Falls Down and Heard Em Say. (not that that's relevant!)
I also like how Kanye's experimented with different sounds and styles and influences. On the other hand Em his has own style going on that works and is popular so I'm not knocking him either.