You could argue that the best guest verse in the history of hip-hop is the one that Chance The Rapper does on Kanye West’s beautiful “Ultralight Beam.”
The song is built around an eight bar loop. (See this post for an analysis of the chord progression.) Chance’s verse goes through the loop five times, for a total of forty bars. It’s not at all typical for a rap song to include a one and a half minute guest verse–it’s almost enough material to make a whole separate song. By giving up so much space in his album opener, Kanye is gaving Chance the strongest endorsement possible, and Chance makes the most of his moment.