"I Love You for Sentimental Reasons" (The Righteous Brothers)

"I Love You for Sentimental Reasons" was written in 1945 by Deek Watson and Pat Best of the vocal group The Brown Dots, a group Watson formed in 1944 after leaving The Ink Spots.  (Although, Best claimed that he alone wrote the song as a love letter to a girl, and Watson just took a writing credit as the leader of the group.)  Whatever the case, it's a song I never tire of hearing, no matter who's covering it or how many times it's covered.  And many have covered it: Nat "King" Cole, Ella Fitzgerald, Sam Cooke, Marvin Gaye & Mary Wells...  They all did great versions.  However, I prefer the Phil Spector-produced version for The Righteous Brothers from 1965.
Released as the B-side of the hit single "Ebb Tide," it's essentially a solo record by Bobby Hatfield (not unlike "Ebb Tide," actually).  As usual, Spector uses his "Wall of Sound" production technique, surrounding Hatfield with swirling strings, plucked harps, thumping bass, heavily reverbed drums, and female backing vocals.  It's no less dramatic than anything else Spector ever produced, but the feel here is even more pillowy and—dare I say it—sentimental; sonically, it's like taking a stroll through a cloud.
And there's nothing quite as soul-stirring as hearing Hatfield hit those high notes on the bridge.  If you've never heard this rendition before, you'll be a convert after you hear him sing Whenever you're in sight.

"Just Once in My Life" (The Righteous Brothers)

"Just Once in My Life" (a 1965 single written by Carole King and Gerry Goffin, and also credited to producer Phil Spector) kind of gets overshadowed by The Righteous Brothers' first and biggest hit, "You've Lost that Lovin' Feelin'."  Although it's obvious that Spector enlisted King and Goffin to write a followup with the same sound and feel as "You've Lost That Lovin' Feelin'," I've always felt "Just Once in My Life" is a superior song.  
(Side note: the writers of "You've Lost That Lovin' Feelin'," Barry Mann and Cynthia Weil, were working on their own followup song, "(You're My) Soul & Inspiration," but felt it was shaping up to be too similar to the former.  So they pushed to shelve it.  Spector vehemently disagreed, which led to Mann and Weil being ousted and King and Goffin being brought in to replace them.  Later, when The Righteous Brothers had gotten their fill of Phil and jumped ship, they went back to Mann and Weil and convinced them to finish writing "Soul & Inspiration," which the group turned into their second #1 single in 1966.)
Production-wise, Spector uses practically the same instrumentation and "Wall of Sound" techniques on both songs: multiple instruments simultaneously playing the same lines, tons of echo and reverb, sentimental strings with orchestral tympani percussion, and angelic backing vocals.  
Performance-wise, Bill Medley lets his soulful baritone float over the lush instrumentation, transmitting every verse as if his whole life depended on it.  And Bobby Hatfield is no slouch either, even though you only hear his distinctive tenor briefly toward the end of the song.  (Again, not that different from its predecessor.)
The biggest difference is the breakdown after the second chorus.  Yes, "You've Lost that Lovin' Feelin'" also has a breakdown after the second chorus—a little song within a song that takes you to another place for a minute or so.  But there's something a little more real, a little more soulful about the breakdown in "Just Once in My Life."  Lyrically, it's not so much a plea for a lover to rekindle a dwindling flame; it's a plea from a financially and emotionally broken man, asking his woman not to give up on him as a human being.  It's a subtle difference.  But it's what makes "Just Once in My Life" a song about personal triumphs and not just love turned cold.