Lots of good ones already posted. Picking only ONE is impossible but I'd have to give strong consideration to those from the Doo Wop era, when many of the greatest love songs were recorded, including, without limitation:
I Only Have Eyes For You (The Flamingos)
For Your Precious Love (Jerry Butler)
Let It Be Me (Betty Everett & Jerry Butler)
Only You (The Platters)
When A Man Loves A Woman (Percy Sledge)
Not from that era, but have to include Crazy Love (Van Morrison) and Ribbon In The Sky (Stevie Wonder).
Oh, and almost every song performed by Nat King Cole.
