If I just HAD to pick 1 U2 song as my favorite, it would be Lemon.
I think those are some of the best lyrics Bono has ever written. According to the book "Into the Heart," the song was inspired by Bono seeing a home video of his mother, and the only color that was still visible was the lemon yellow of her dress.
The music itself is glorious.. those shimmering, echoing chords.. that low, brooding cello. It compliments the lyrics and the subject matter as perfectly as any U2 song, and maybe just a little bit better than most. Bono seems to use his voice more like it was one of the instruments in this song. It is so subtle and nimble, wavering just the right amount at just the right time. And the "solo" is simply transcendant. Such a simple melody can bring so much pleasure.
The first time I heard it.. well, it was love at first lemon.
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"..and I feel like I'm swimming out to her.."