I had to vote for Pop (2 votes ahead now!). That album had such a deep impact on me as a 14 year old boy, I've spent the last 20 years trying to find music that gives me the same buzz that Pop gave (and still gives) me and nothing else compares (though to be fair, some bands have come very close over the years!).
I do LOVE Zooropa though, particularly the title track, Lemon, Some Days, Dirty Day and The Wanderer. I still remember the first time I heard the album though and it was a definite WTF moment
My first U2 album purchase was 'Rattle And Hum' and I was impressed enough as a young boy to investigate further so I borrowed 'Zooropa' from my local library- The album artwork and the music really confused me at the time, where had the cowboy hats gone and why was Bono singing about washing up liquid? It makes a lot more sense now though