There's not a single rapper I am a fan of, they're all very inconsistent in the quality of their works.
Personal faves are:
Hip Hop Saved My Life - Lupe Fiasco (I mean Stack That Cheese)
Playing with fire/Nine Thou - Styles of Beyond
When I'm Gone/Mockingbird/Stan (/w Dido EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE)/Without Me/No Love - Eminem
Never Change - Sam Ock
Ridin' Dirty - Chamillionaire
And a very personal one:
Smile Through It All - J. Reyez
And maybe:
Velocity - Tonedeff
EDIT*
And shameless advertising to this cover of Lighters by em, 5'9 and that guy who got killed by a grenade:
http://www.youtube.com/watch?v=r9k8WTyDEg4
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What matters to me for Rap:
1. Fluidity
2. Enunciation and Accent (If the rapper has a muffled nasally voice like my own, please, please, shut up, and stop rapping)
3. Artistry and word play
4. Message
5. Baseline and Beat
6. Chorus
7. Tempo (I prefer fast paced rap)