October is the worst, it's not even a well-lit or sharp photo. I don't know how this is even debatable.
HTDAAB is indeed lame because they're sitting down and it makes them look geriatric (aside from Edge who at least looks like a badass East L.A. gang member), but the actual photography is nice, plus I really like the font/typography reminiscent of a web address and the red slashes on the black border.
I think if you're judging solely on the pose you're really not taking the whole cover into account because it's not only about Corbijn but Averill's work as well.
The Joshua Tree obviously has a great photo but I also love the widescreen framing and all the black space. TUF has great coloring with the burgundy as well as the blurry, impressionistic photo that is a great representation of the music.
Achtung is a clever idea, riffing on Exile on Main Street while reflecting on the chaotic atmosphere of Berlin/the new EU, and the bursts of color giving the band a fresh look.
Ultimately I think I have to go with War as the best. The photo is so striking, so perfect for the album's themes, and I love the self-reference to the Boy cover. Plus that bold red lettering pops so well off the black & white.