Joni Mitchell’s final line in “Big Yellow Taxi” shoes off her chops, stretching her range from quite high to quite low.
I’m pretty sure, though, the laughter (which came across to me as contrived, forced) was a separate track. (more of a novelty one, this example)
Brian Wilson gets absolutely stratospheric with the sublime title lyric from God Only Knows.
Patsy Klein’s opening line in Crazy for some reason always ‘gets me’.
Doesn’t matter what genre. Curious to hear operatic contributions.
First line of On a Plain, by Nirvana
I’ll start this off without any words …
Maria Callas singing “Habanera”: FF to her breathing in at 6:06, and then listen to the volume and exquisite control she has on “prends garde a toi.”
Or just listen to the whole thing. Best operatic voice of the 20th century.
Annie Haslam. The last word of “Song of Scheherazade.” (The last line of the song is the name repeated a dozen times or so as part of a musical phrase, each repetition adding more instruments, then shifting up a third, then Annie hitting a note only dogs and bright, hip people can hear.
2nd choice: Jay Black (Jay and the Americans) holding the word “time” for an insanely long period of time (heh!) during any live performance of “Cara Mia.”
I’m not sure if it was multi-tracked or not, but the long notes at the end of Jeff Buckley’s version of Hallelujah are pretty impressive (even if the song is quite overplayed at this point). Around 6 minutes: https://www.youtube.com/watch?v=y8AWFf7EAc4
For Nirvana I quite like the end of “Where Did You Sleep Last Night?” from their Unplugged set. Around 4:45 here: https://www.youtube.com/watch?v=hEMm7gxBYSc
The opening line of Layla - Derek and the Dominos
What will you do when you get lonely
The pain and angst of his unrequited love to be spilled out in the rest of the song is as powerful as any bluesman whom he’s been trying to emulate for decades.
Speaking of blues(woman), Ball and Chain - Janis Joplin
And I say, oh, whoa, whoa, oh, that cannot be
Wow, she does impress with that last note. I was thinking of something else by her - the chorus of, “Carpet of the Sun”, the live version:
What she does on the words, “Soft and sweet” and “Of oceans mountains deep”,wow.
Interesting - I found this Clapton lyric, from “Bell Bottom Blues”, to be even more heart-wrenching.
Any line you like out of this. Maybe, “I was born by the river, in a little tent, and just like the river, I’ve been running ever since,” at the start. Or “There’ve been times when I thought I couldn’t last for long, but now I think I’m able to carry on,” at the end.
Well, it’s got to be Annie Ross. Some people dig “Twisted,” but for me the most impressive was “Jackie”.
Isn’t that Carl?
Roy Orbison on “Only the Lonely”:
But that’s the chance
You gotta take
If your lonely heart breaks
Only the lonely
Ah, true - Carl was definitely the lead vocalist for that one (and written by Brian W. and Tony Asher). Interesting that in the video it has Brian (with acoustic guitar) singing lead. (heh, or lip-syncing?)
Maybe my favourite Paul singing line (non-Beatles!)
From No More Lonely Nights.
Linda Ronstadt - Do What You Gotta Do - the last line “Come on back and see me when you can.” Wow.
That’s Carl and Bruce Johnston at that mic. Brian isn’t in that video although he sings backing vocals on the track. Mike Love, seen in the video, did not sing on the recording.
Indeed, right again. 
Blaming it on all that damn hairless baby fat.
Opening line from Janis’s Piece of My Heart has just a little bit of oomph or something going on.
Impressively sung lines? This is a candidate:
Every time she sings “breath” in Teardrop. Probably tied with it is the scat at the end of Birthday by the Sugarcubes but that isn’t really a “line” per se as it isn’t lyrics.
I’m always struck by the way Eddie Vedder sings the last lines of “Black” by Pearl Jam. I can feel the rage and loss that’s being put out there:
I know someday you’ll have a beautiful life
I know you’ll be a star
In somebody else’s sky
But why
Why
Why can’t it be
Oh can’t it be mine