Your ONE all time favorite love song?

The Flying Burrito Bros. Hot Burrito #1 aka I'm your Toy.
You can hear Gram Parson's heart breaking as he sings.I have a couple more but you did say one...sigh.

how about joe tex (ca. 1965) : you got what it takes (to take what i got).

the cover by marti jones with don dixon (her husband) from the "my long haired life" album is great - my wife and i used this arrangement for our first dance.

Love songs also exist in personal context. When my wife was pregnant with our first child (a boy), we had our first quality system (Thiel, Hafler, Thorens). We danced and dreamed and cried over our happiness to Loggins & Messina Danny's Song. Man, that was a love song to us. Maybe I'm OP, but nice to recall that song, those feelings. He's now 22....

Autumn Leaves

BTW regarding a previous post "Angel Baby" was recorded by Rosie and the Originals not Ruby and the Romantics

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.