Roll over Beethoven,tell Tchaikovsky the news- Chuck Berry
From Bob Dylan’s Motopsycho Nightmare:
I fell down
To my bended knees,
Saying, “I dig farmers,
Don’t shoot me, please!”
He cocked his rifle
And began to shout,
“You’re that travelin’ salesman
That I have heard about.”
I said, “No! No! No!
I’m a doctor and it’s true,
I’m a clean-cut kid
And I been to college, too.”
or
I’m a third generation counter-spy
for the CIA and the FBI
C’mon, do the third war rag
by Bob Martin
You know, I hate to say it, but I never got that humor until I just saw it written down. Thanks!
My submission is kind of a cheat, because the line is funny only because of what comes before it. Actually, there are two of them, both from Arlo Guthrie’s Alice’s Restaurant:
Yes sir, officer Obie, I cannot tell a lie: I put that envelope under that pile of garbage.
Obie came to the realization that it was a typical case of American blind justice, and there wasn’t nothing he could do about it, and the judge wasn’t going to look at the twenty seven eight-by-ten colour glossy pictures with the circles and arrows and a paragraph on the back of each one explaining what each one was to be used as evidence against us.
Joy to the world was a beautiful girl
But to me Joy meant only sorrow.
Harry Nillson
I’m certain I’ve spelled his name wrong, but I LOVE that line.
“But you forgot what I was saying… 'cause YOU’RE AN ASSHOLE!” – Frank Zappa in “Broken Hearts Are For Assholes”
“God knows what he’s doing. After all, he wrote this book here. And the book says He made us all to be just like Him, so if we’re dumb, then GOD IS DUMB, and maybe even a little bit ugly on the side!” – Zappa on religious fanatics in “Dumb All Over”
“I’m sure we all agree that we ought to love one another, and I know there are people in the world that do not love their fellow human beings, and I hate people like that.” – Tom Lehrer in “National Brotherhood Week”
“It is a sobering thought, for example, that when Mozart was my age, he had been dead for two years.” – Tom Lehrer in “Alma”
“When they see us coming, the birdies all try and hide, / But they still go for peanuts when coated in cyanide.” – Tom Lehrer in “Poisoning Pigeons in the Park” (OK, so that one’s not technically a one-liner. Tough.)
“Obie, Did you think I was gonna hang myself for littering?” - “Alice’s Restaurant”
“My name is Sue! How do you do?!? Now you gonna die!” - “A Boy Named Sue”
Almost any line from the John Prine classic “Please Don’t Bury Me”, but my favorite part: " Send my mouth way down South to kiss my ass goodbye"
The way Warren Zevon mumbles the line “I don’t want to talk about it.” in his version of “Poor Poor Pitiful me” cracks me up everytime.
From “L’america”, by the Doors:
“C’mon people don’t you look so down
you know the rainman’s comin’ to town
he’ll change your weather, change your luck,
and then he’ll teach you how to f… find yourself.”
“She lowered her standards by raising her glass, her courage, her eyes, and his hopes”
From Madeira, m’dear by Flanders and Swann. who have written far too many other hilarious lines to even begin quoting them all…
'Cause I’m stranded all alone in the gas station of love
And I have to use the self-service pumps…
“Weird Al” Yankovic, “One More Minute”. But you knew that already.
The Police: “On Any Other Day”
“…and when the wombat comes
He will find me gone…”
Of course just about any line from Tom Lehrer or Weird Al Yankovic is going to be funny.
So would lines from just about any Monty Python song or Alan Sherman song or Ray Stevens song or… well, you get the idea.
Of course, it’s not my OP, so my added rule that the line should be from a song that is otherwise “not funny” might just be my OCD talking. 
I’d even think that the “Boy Named Sue” line is stretching it, as it is in keeping with the tone of the rest of the song, but I suppose most don’t expect Johnny Cash songs to be funny (although anyone who listened to Dr. Demento this week knows that Johnny Cash did, indeed, do several funny songs).
I nominate Meatloaf’s “Paradise by the Dashboard Light”, when it says, after the whole big set up including Phil Rizzuto doing play-by-play and about swearing to love her until the end of time:
“So now I’m praying for the end of time,
To hurry up and arrive,
'Cause if I have to spend another minute with you
I don’t think I could really survive.
I’ll never break my promise or forget my vow
But God only knows what I could do right now.
I’m praying for the end of time
It’s all that I can do
Praying for the end of time so I can end my time with you!”
There’s a song by Tony, Tone, Toni (or is it Toni, Tone, Tony), the title of which escapes me right now. There’s a part that cracks Mr. Tech and me up each time we hear it (they’re in a club, and there’s much drinking going on, from what we can gather):
Who is your friend?/
She don’t look nice/
But I know she will/
Later on tonight.
“Spending warm summer days indoors, writing frightening verse to a buck-toothed girl in Luxembourg”
The Smiths : Ask
How can I leave this behind?
- Spinal Tap
I think that’s “underwear.”
“Then there was that time you drove that little car down Sunset Boulevard at three in the morning doing ninety miles an hour in the thirty-mile zone and blamed me when you got that ticket.”
– from the gloriously terrible Rosecrans Boulevard, written by Jim Webb (sort of as a follow-up to his Macarthur Park) and recorded by Johnny Rivers. The song didn’t rhyme and was just Godawful. The line, BTW, was sung, not spoken.
For a better-known option, I hate Barry Manilow with a passion, but I can’t stop smiling when he sings (in Copacabana):
“She lost her youth and she lost her Tony,
Now she’s lost her mind!”
I think it’s the pizazz he puts on the last three words that makes me lose it.
" But not a real green dress, that’s cruel" -If I had a million dollars
And They Might be Giants:
“What’s that blue thing
doing here?”
from Fingertips (although that really is the whole song).
“I know it stinks in here, because I’m the shit!”
–Kid Rock, American Badass (I think)
Line cracks me up all the time.
Kid Rock has another good one: “I’m in it to win it like Yzerman, can drink about 15 Heinekins”
Don’t remember the name of that song though.
/not really a kid rock fan
David Allen Coe - Perfect Country and Western Song
I was drunk the day my ma got out of prison.