I think you know the answer to that -- it's the "Summer Wind,"* a late masterpiece (the latest-written song on this list) from the Sinatra/Riddle/Mercer body of work:
Oh, Nelson. Thank you. That was lovely. It swings, but at the perfect tempo, slowish but not too slow.
Johnny Mercer's words, written "along the same theme" as the original German lyrics by Hans Bradtke, reveal a songwriter at the top of his game. All those vivid visual images -- of golden sand, painted kites, a blue umbrella sky -- make it special. But it's the emotional content, the story of love and loss, so deftly rendered, that makes it great. The loss is grievous but the lyrics never approach the maudlin,* though certainly we feel the man's pain -- he lost her, and his lonely days and nights "never end":
The summer wind came blowin' in from across the seaDitto, of course, for Sinatra, whose maturity adds something essential to the performance. The year was 1966, making him just about fifty when he recorded it. I can't imagine a young person getting it right. It feels like a song by, for, and about grown-ups.
It lingered there, to touch your hair and walk with me
All summer long we sang a song and then we strolled that golden sand
Two sweethearts and the summer wind
Like painted kites, those days and nights they went flyin' by
The world was new beneath a blue umbrella sky
Then softer than a piper man, one day it called to you
I lost you, I lost you to the summer wind
The autumn wind, and the winter winds they have come and gone
And still the days, those lonely days, they go on and on
And guess who sighs his lullabies through nights that never end
My fickle friend, the summer wind
The summer wind
Warm summer wind
Mmm, the summer wind
Coincidentally, Bob ranks "Summer Wind" in the number four position, too. Mark hasn't written about it yet but I'll be very surprised if he doesn't. I'm counting on him to discuss Mercer's intricate, wonderfully effective rhyme scheme. (My favorite rhyme might be sighs/lullabies. Feels so good.) Watch the sidebar for a link to his essay when it comes. I'll update this post with the link, as well, and add one or two choice excerpts.
*There's an earlier version of the lyrics that actually does descend into self-pity, thereby ruining the entire effect of the song. (I found it in this book.) Will Freidwald on that:
"Summer Wind" was recorded that year  by Perry Como in a deadly dull, teutonic hillbilly treatment whose only plus is that the touching second chorus (unsung by Sinatra) is included. (p 273)I guess one fan's "touching" is another's maudlin. I for one am very grateful that Mr. Mercer re-wrote his lyrics. If you really want to hear Mr. Como's version (and you probably don't) you can find it on YouTube.
(I credit this Mastercard commercial featuring the song for sparking my interest in Sinatra about a decade ago. I heard that voice and that sound and needed to hear more.)
*I mixed up my #3 and #4, now corrected. Like it matters. :)