What: Radiohead's meandering, transcendent ballad "Let Down," from the band's 1997 masterpiece album OK Computer. Here's a video which helpfully spells out the lyrics to the song.
Why: If Radiohead were reading this—and really, they're much too hip for that—they would be shaking their heads now. From 1993–1997, the period most defined by the term "Britpop," Radiohead was pushed to the fringes by the champions of the too-cool-for-school set. Like all British movements, Britpop was a revolution against something American, in this case grunge. Radiohead couldn't be bothered with other people's causes, but they could, for the time, be counted on to churn out the occasional phenomenal pop song, such as "Creep" or "High and Dry". (Naturally, these turn out to be the songs that singer Thom Yorke detests. Remember, they're very hip.) In 1997, the band released the epic OK Computer, known for its dystopian singles "Paranoid Android" and "Karma Police". Jammed right in the middle was "Let Down," an undulating dirge of loneliness and insecticide, featuring a guitar line by Jonny Greenwood in a different time signature than the rest of the song. No matter, it's pop perfection. On any other album of the time, this would be the chartbuster, in rotation between singles by Blur and Ocean Colour Scene. On this album, it seems a bit out of place. But after a decade of steady listening, "Let Down" is the standout song, not just of the album but the entire period of British alt-rock.
Impact: Considered for OK Computer's lead single, "Let Down" was never released as a single, but still reached #29 on the U.S. Modern Rock charts. Meanwhile, the album changed all the rules. Wedging itself between Nirvana's Nevermind and Beck's Odelay on the must-owns of the decade, OK Computer made it okay for bands to try, however hopelessly, to recreate the sonic supremacy of the Beach Boys' Pet Sounds. As Britpop faded, Radiohead left for stranger planets, producing a decade of oddities until alighting back on Earth with 2007's mindblowing In Rainbows. About then, the band released an expanded OK Computer, with additions like "Palo Alto" making the best album of the '90s even better.
Personal Connection: I mentioned the band was pushed to the fringes of the post-Madchester scene. It took a breakout on the West Coast for Radiohead to become, well, Radiohead. It strikes me that my friend and colleague Jenny Bendel, Radiohead's U.S. press agent at the time, never gets enough credit for breaking them in the States. There's a chance they would have gone stratospheric without her, but they didn't, so thank Jenny the next chance you get.
Other Contenders (limited to British artists recording from 1993 to 1997, regardless of when these songs were released): Mike Hedges' mix of the La's' cowbell-clangy "Come In, Come Out"; Suede's logo-intense video for "Beautiful Ones"; the Sneaker Pimps' graceful "6 Underground"; Oasis's last great power-pop assault, "D'You Know What I Mean?"; "Bitter Sweet Symphony", the Verve's totally unacceptable (and awesome) theft of the Rolling Stones' "The Last Time"; Placebo's big reach backward, a cover of Kate Bush's "Running Up That Hill"; the most-definitely-not-safe-for-work revelations of Britpop standard-bearer Robbie Williams's ode to Robbie Williams, "Come Undone".