I actually think TBA was more of a return to form before AG. TBA wasn't necessarily RD level of lyricism but it was awfully close. I think Jay-Z was at his most complete on that album. But The Blueprint is more of a classic in the sense of influence. And yeah AG stays underrated. Great album.
The Black Album has a lot of brilliant moments but I think a few of the tracks on there are just ok, to just wack (Change Clothes, Dirt of Your Shoulders, Lucifer, Justify My Thug). Just wasn't that consistently great throughout for me to call it a classic. Lyrically Jay definitely brought it though, I agree. I say American Gangster is like a return to form because his album before is named a lot in talks about Jay's worst album (Kingdom Come).