Your ONE all time favorite love song?

"You're just in love" - Irving Berlin

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.