I don't think "MacArthur Park" quit fits the definition of a seriously intended song lyric...but I could be wrong, I suppose.
"Having My Baby" is indeed a killer. Hard to beat. Curiously enough, Paul Anka's very next hit (again in a deathless duet with Odia Coates) was "One Man Woman"..."you caught me foolin' around with somebody new, you caught me foolin' around, now I'm losin' you...". This is ironical. Apparently his interest in the new mother and child situation flagged rather rapidly...if you can believe the songs.
Don't nominate William Shatner doing "Lucy In The Sky With Diamonds" or anything else either...those cannot be called serious renditions of songs.
Here's another nomination: "You And Me Against The World" I think it was by Helen Reddy, wasn't it?