In order of personal preference:
1. Achtung Baby
2. The Joshua Tree
3. The Unforgettable Fire
4. Wide Awake in America (EP)
7. All That You Can’t Leave Behind
10. Rattle and Hum
11. How to Dismantle an Atomic Bomb
13. Songs of Innocence
14. No Line on the Horizon
I should note that a) the difference between positions one and two is so small as to be almost invisible, b) the only album here I think of being genuinely bad is No Line, c) although I fully note that Rattle and Hum (where the “mediocre” level begins on this list) is the album where U2 was the most egregiously full of itself.
Provide your own thoughts and rankings in the comments.