Jay-Z is the worst rapper in the history of rap. I swear I think I can rap better than him on my worst day.
Jay-Z is far from the best lyricist, but I wouldn't ever refer to him as the worst rapper. He's certainly taken a turn for the worse since coming back from his "retirement," but even his newer songs are better than a lot of shit I've heard (Soulja Boy, for instance). And when you look at his entire body of work, I find it hard to justify stating that he's the worst rapper in the history of rap.
Eminem's verses kill him on this particular song, but this is just one example of why Jay-Z is not the worst:
Or maybe you need to be reminded of Jay-Z prior to his retirement and unretirement?
At any rate, from what I've heard of the album, it's pretty shitty. Repetitive samples throughout the majority of a song. Repetitive beats with no real variety or distinctive verses, choruses, and bridges. Nothing stands out lyrically. Beyoncé is annoying, Frank Ocean is not going to make it as an R&B artist, and no other collaborator on the album stands out as remotely popular/talented (aside from Otis Redding, who's been dead for 44 years).
To me, it feels like Kanye threw some shit together. It's as if he assumed that he could do no wrong when it came to making beats, so he just started synthesizing shit while high. Then he took the 16 bars he just shittily created and put them on loop for 4 minutes. Cue some unimaginative lyrics about black empowerment and BAM! You've got Watch The Throne.