Damn it, Defiant! lol j/k that's a real tough one though...
I'm going to have to say Illmatic because of it's significance with East Coast hip-hop, the lyricism is untouchable at least at that time, the production was top notch especially having DJ Premier, Q-Tip, Pete Rock and Large Professor handling the production that was unheard of at that time (alot of rappers at that time only used one producer) it revived the Queensbridge rap scene, and it's regarded as one of the most influential rap albums of all time...now The Eminem Show may be held in that same regard somewhere down the line but as of now it's not in my opinion...Bottomline- Illmatic had more of an impact...
Mat, as far as Coolio goes...I only like two of his songs and that's "Fantastic Voyage?" and "Gangsta's Paradise" other than that I don't see him as one of the "greatest" or "top" rappers...