Well I dont know about nas I mean of course hes definetly in top 5 but......hmmm......But thers Eminem ( I dont like him but admit hes ill) thers probly some more I cant think of but in my opinion the most more known skilled lyricists right now are:
Nas
Eminem
Chino XL ( if u never heard him download his shit HES ILL)

and jay but he jus retired