Your ONE all time favorite love song?

Can't pick just one. "The First Time Ever I Saw Your Face".
Johhny's Cash's rendition is something to behold. It's on
"American IV: The Man Comes Around", his last recording.
"True Companion" from Mark Cohen's debut album.

One would be tough; categorically, though:

Old songs:
"Chances Are" - Johnny Mathis

"New" songs:
"Somewhere, Somehow" - Michael W. Smith and Amy Grant

The "I'd never believe they'd write a love song, much less a good one" category:
"Different Strings" - Rush

Reasons for waiting. Jethro Tull, Stand Up.

"At this Moment"--Billy Vera and the Beaters, "To Make You Feel My Love"--Bob Dylan or Garth Brooks, "Color My World"-Chicago, "I Will Always Love You"--Dolly Parton

There's a whole story behind this song, but let's say its a true gut felt love song he wrote for Susannah, Wendy Melvoins twin sister.

Its his best. What a climax.

Off of Sign "O" The Times (1987) which is overdue for a remaster. Vinyl is waaaay better if you can get your hands on that record, worth a spin for beyond Purple Rain appreciation.