Not only one of great first lines, but considered to be some of the most evocative lines in a song ever (from a guy who wrote these in the hundreds and thousands):
"Your lips were like a red and ruby chalice, warmer than the summer night, The clouds were like an alabaster palace, rising to a snowy height." - Midnight Sun lyrics by Johnny Mercer
...and it was retrofitted to a previously written instrumental piece with a non-song-like melodic line to boot (by Lionel Hampton & Sonny Burke).
At words poetic, I'm so pathetic/That I always have found it best,/Instead of getting 'em off my chest,/To let 'em rest unexpressed,/I hate parading my serenading/As I'll probably miss a bar,/But if this ditty is not so pretty/At least it'll tell you/How great you are.
The greatest first lines for a MUSICAL COMEDY song were penned by Ira Gershwin (to George's tune) for Fred Astaire to burst into song in DAMSEL IN DISTRESS:
"If I should suddenly start to sing, Or stand on my head, or anything, Don't think that I've lost my senses. It's just that my happiness finally commences..."
(Things Are Looking Up).
In this regard, honorable mention goes to Cole Porter:
"I feel the sudden urge to sing The kind of ditty that invokes the spring..."
(It's Delovely)
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Perhaps the greatest couplet to open any song was this one, made famous by Fats Waller, (sorry, I don't know the songwriters):
"Up in Harlem, at a table for two, There were four of us: me, your big feet, and you."
