Lyrically, I think Eminem is on top.
Firstly, he can do it freestyle/spontaneous and his words come out like genius. Not all rappers can do that.
Second, he can have serious songs or he can have HILARIOUS stuff. Or songs that balance both.
Personally, I love his lyrics in "I Remember", a song where he is trashing Everlast...especially this verse:
What's up with you?
I never fucked with you, why would you fuck with me?
Knowin I could rap circles around you, what, are you as nuts as me?
Plus I can sing better than you and I don't fuckin sing
And probably play guitar better, and I ain't never touched a string
But I ain't mad at you, I'd hate me too if I was you
I'm what you used to be, shit you was me in '92
So everytime I write a lyric, I'mma think of you
And maybe that will help me know what it's like to sing the blues
