In this world, it's what have you done for me lately combined with "legend" status plus a little bit of what other people say.
I couldn't recite two KRS versus word for word, yet I'd be inclined to say he is a better lyricist than....Joe Budden, in which whom I've probably heard 95% of what's available to hear. That is illogical and stupid. That's the legend status.
A personal top 5, should be just that....who you like to listen to. That could range from Gucci Mane and Slim Thug to Currency and Mos Def. I think that has a lot to do with your lifestyle and your mentality at any given time, coupled with your understanding of what that person is saying.
I don't have Nas in my top 5, but I think he's a great lyricist and is brilliant story teller. I do not listen to Nas on a daily basis. He barely gets play in my car. Perhaps if I had to catch a subway to work and was more prone to headphones, the stuff I listened to daily would be different (no subwoofers). I listen to a lot of Joe Budden, Jay-Z, Jadakiss, Young Jeezy, and Drake, so they'd have to be in my top 5. Big Sean is getting there. Top 5 of all time.....well no, but 12 years ago, Mac from No Limit was in my top 5, so was Nas because of how much I listened to "I Am"