Well, Tupac and Biggie died young, so they automatically have seats in the greatest rappers of all time list. Although, Pac was actually a great poet.
My favorite rappers (and best IMHO) are:
Eminem. Yeah, he's on top AGAIN, so there will be a lot of naysayers, but the truth is, the guy is a great lyricist, and he has the ability to make me laugh out loud, and send shivers down my spine. He can be very dark, and go totally opposite side of the spectrum. My favorite.
Slug (of Atmosphere). He may not go down as one of the best, because honestly, he's kinda stuck in Eminem's shadow, but his lyrics and delivery are top notch IMO. He's right up there with Em.
Lupe Fiasco. Another rapper who dares to be different. His flow is nice and his sound is completely original and refreshing.
Kanye West. He's an egomaniac, but a great producer with some of the best hip-hop albums of the past decade. His flow isn't great, but his wordplay is superior to most.
Jay-Z. Yeah, he may be a "biter" at times, but when he's not biting, he's great. The Black Album is one of my favorite rap albums of the 2000's. He will undoubtedly go down as one of the best.
Eyedea. As for battle rappers, he's one of the sickest.
Busta Rhymes. Despite his decline in popularity, he too is a name you will see on any list pertaining to rap legends. His sound has always been unique, and he can rap so fast a listener has to pay close attention to keep up, or slow it way down for a love jam.
The Wu-Tang Clan. These guys will always have a following. Method Man is a name most people who listen to rap are familiar with. Redman and the late O.D.B. are pretty renowned too.
Mos Def. Another lyrical genius. Nuff said.
Nas. Like Jay-Z, he's had his ups and downs, but through it all, he's maintained enough respect to land him in the hip-hop hall of fame.
Black Thought. The Roots win the critics over time and time again, and have developed somewhat of a cult following. Black Thought is deep.
Del the Funky Homosapien aka Delton 3030. Some people know him from the Gorillaz (Russell), but others no him from his indie rap. Ice Cube's cousin is pretty far out there. Lyrically, one of the best out there today.
Ludacris used to be one of my favorites too, but he's gotten pretty bad as of late, doing songs with Jesse McCartney & Justin Bieber. Makes me wonder, "How Low Can You Go?"