Too me every era in hip hop has their own "legends" and you can't necessarily compare different eras. To me most 80's and early 90's stuff sounds kind of boring now and I'd much rather listen to Ross or Game or Jeezy even if they're perceived as being lesser lyricists but I can still respect both groups of artists as "legends".
There's only 5 artists that seem to transcend the eras as of now: Tupac, Big, Jay, Nas and Em. Every other "legend" was only a legend by their eras standards, and if they had debuted in another era their music wouldn't of had the same impact IMO.
Basically here's how I would divide up the eras and their respective legend:
80's/Early 90's (Basically pre-Illmatic):
Rakim, Kool G Rap, Jam Master Jay, Eazy-E, Slick Rick, Beastie Boys, Big Daddy Kane, LL Cool J, Ice Cube
Mid 90's:
Tupac, Big, Nas, Jay-Z, Snoop Dogg, Dr. Dre, DMX, Rae, Ghost, Outkast
Late 90's/early 2000's:
Eminem, 50 Cent, Cam'ron, Common
Mid 2000's:
Rick Ross, Young Jeezy, Game, Kanye, Lil Wayne (realize he debuted in the 90's but mid 2000's is when he really took off)
Late 2000's/early 2010's:
Drake
Too early to tell but someone like Kendrick Lamar or J. Cole probably have next
Again not an all inclusive list, I'm probably missing a handful, but pretty close.
Of course there are regional legends like E-40 in the Bay Area, and there are artists that are legends to certain groups of people within the genre, like Kid Cudi to his rather large, rabid fan base and I respect them too.
The most frustrating thing is when nostalgic old heads put music from 20 years ago on a pedestal like it can never be touched. The thing is today's artists don't want to touch it, the musical climate and people's preferences have changed.