Also, he probably doesn't belong with those guys, but maybe best all around songwriter in hip-hop would be Andre 3000.
If we're being completely honest to the profession, the best lyricists are all rappers/hip hop artists considering the average length of their verses compared to other musicians. Old school people won't like it, but it doesn't change the fact that they have to keep your attention much longer and to a harder rhythm (or an actual one unlike some musicians do).
Back to the idea of rock/older musicians though, this conversation needs Don Henley and John Fogerty.
A proper measure for how good/great a songwriter is the number of times that song gets covered by other artists/bands. This list is endless of these people but I do find it laughable that we have some believing their favorite hip-hop/rap artists falls into this category.
Lennon/McCartney
Willie Nelson
Burt Bacharach
Pat Metheny
Miles Davis
George Gershwin
Ira Gershwin
John Williams
Elton John/Bernie Taupin
Cole Porter
Simply too many to list
Tupac, Eminem, Nas, etc. would all like words with you on how often they get sampled/covered. In fact, if we're talking pure numbers (which is what you imply), hip hop people get their beats and songs sampled more than any other, so, by your own logic, the best hip hop artists belong at the very top if your list.
Covered by whom? Hip hop artists covering each other doesn't count, but I've certainly heard a number of others covered by samples in hip-hop/rap