Cocker is probably the best Beatles interpreter alive, having covered several other songs. But its this song, which was used as the theme song for The Wonder Years, that really proved how good he is. Turning the poppy number into a rousing gospel number, Cocker gave the song an emotional depth that stirred the soul. It also set the standard for what a cover song should sound like, and gave Cocker a hit in his own right.
2. "Hey Jude," Mutato Muzika Orchestra
"Hey Jude" is already a seminal Beatles song; its one of their best, and improving upon it is a herculean challenge. But Mutato Muzika Orchestra did just that. By stripping the song of its lyrics and adding beautiful orchestrations to the classic melody, the song becomes something entirely new while remaining instantly recognizable. It should come as no surprise that Wes Anderson chose the song for the opening credits of The Royal Tenenbaums, since it manages to match the film's unique mix of austerity, whimsy, and longing.
3. "Real Love," Regina Spektor
Okay, so this technically isn't a Beatles song in the strict sense, but it is from John Lennon's solo career, and I had to include it. In 2007, Amnesty International commissioned a variety of artists to do covers of John Lennon songs for Instant Karma, a compilation in which proceeds would go toward ending genocide in Darfur. While most of the songs on the album were standard or lackluster, they obviously saved the best for last: Regina Spekor's cover of "Real Love" is a haunting, powerful, emotional rendering of the song, using only her voice and piano to create a lovely spare sound. The result is a heartbreaking celebration of finding "the one."
4. "Because," Elliott Smith
"Because" was always a very ethereal song, a drug-induced meditation on existentialism. But Smith manages to make it even more unworldly by stripping the backing track, leaving the majority of the song a cappella. Smith's version is even more psychedelic than the original, and the atmospherics emphasize the existential themes of the song. Suddenly, what was once a sonic LSD trip becomes a powerful experience.
5. "I Want to Hold Your Hand," Across the Universe Soundtrack (and Glee Cast, later)
With the exceptions of Journey's "Don't Stop Believin'" and Madonna's "Like a Prayer," Glee hasn't been much of a producer of definitive versions of a song; most of the songs on the show are simply karaoke versions of the original. However, in the episode "Grilled Cheesus," the show managed to create a strong emotional moment by turning upbeat love song "I Want to Hold Your Hand" into a longing ballad of hope, sung by Kurt (Chris Colfer) about his comatose father. Its also a perfect representation of the show's earnestness, even though the show often pretends otherwise.
EDIT: Thanks to Mad Hatter for pointing out that the above song was actually first featured in that state in Across the Universe, and should therefore be credited as such. I'm really embarrassed by this, since I'm such a huge fan of the film and can't believe I didn't catch that earlier. The entry is still the same, but I've replaced the video now with a better one, and I apologize for such an egregious error.