I've got to go with Zooropa by a fair margin.
In my eyes, Zooropa has some of my favorite U2 songs (Zooropa, Lemon, Stay), and every other track on it is either great or good (Babyface and Dirty Day being merely "good"). Plus, it's completely different from anything else in the U2 catalogue, it was their most experimental album overall, and it's extremely cohesive - it almost works as somewhat of a concept album. It's like some kind of crazy, spacey dream. It's amazing.
Pop is good. I don't see it as the masterpiece that some people see it as, but I really like some of the songs on it. Mofo and Last Night on Earth especially are really good. Please is also really good, but I can't help but feel that it was slighted production-wise, kind of like Dirty Day on Zooropa. It could have been done much better on the album and therefore doesn't jump out at you as much as it should when you listen to it. I actually really like most of the songs on Pop now that I think about it. It's just that string of three towards the end that kill it for me - that is, Miami -> The Playboy Mansion -> If You Wear That Velvet Dress. I cannot get into any of these songs and I find them to be three of the worst songs in U2's catalogue. They're boring and have extremely questionable lyrics (except Velvet Dress, which I will concede has pretty good lyrical moments). I actually like Velvet Dress a little bit on its own, but it's still kind of boring, and since it's placed after two other boring songs on the album, it really doesn't work for me at all. I appreciate what U2 was trying to do here, but I don't think they quite pulled it off. If they had more time to produce the album and work on the songs a bit more, I feel that I might like it a bit more, but as it stands now it's merely a decent U2 album to me.
So yes. Zooropa owns this one for me.