I think the only good album they had after Joshua Tree was Achtung Baby, but I agree that U2 peaked with the Joshua Tree album in 1987 and their career after was largely undistinguished. I remember the top 5 albums of 1987 which were Joshua Tree, Document by REM, the Smiths' Strangeways Here We Come, which was fantastic, Sign of the Times by Prince, and Kick by INXS. One could argue that each of those albums was the best by each of those artists.