^ flow is how liquidy they sound... if they fit the beat... if they can control their breath properly... it has nothing whatsoever to do with the actual content of the lyrics...

I judge rappers in 3 categories:
Flow
Delivery
Lyrics (split into wordplay and rhyming)

id rather have an awesome flow and average lyrics than awesome lyrics and a crappy flow

anyway, answering your question... the most lyrically skilled MC's, in no order:

Rakim
Canibus
Redman
Nas
Big L
GZA
Eminem
Cormega
Method Man
KRS-One