Rubber baby buggy bumpers - Martin Gardner
Suzie Shelby sits shining shoes. Suzie sits and Shelby shines and Suzie shines as Shelby sits.
(I can’t even say it once)
Try The sixth sick Sheik’s sixth sheep*'s** sick.*
Makes it so much worse.
My brother taught me this one, and I have never heard or seen it anywhere else, so it could be his own creation:
She stood on the balcony inimitably mimicking him hiccupping while amicably welcoming him in.
-Tcat
A review of the ones mentioned in this thread makes it seem that a lot of the toughest ones employ a play on the sibilant ‘s’ sound.
What makes ‘s’ so complicated?
two witches were watching two watches. Which witch watched which watch?
I try and I try, yet I can’t help saying “shits” instead of sits. 
That one’s impossible!
A big black bug bled black blood.
Moses supposes his toeses are roses.
But Moses supposes eroneously.
But Moses, he knowses his toeses aren’t roses.
As Moses supposes his toeses to be.
Eleven benevolent elephants.
A man said “You’re mad!”
I said “Mad?”
He said “Yes.”
I said “Who?”
He said “You.”
I said “Me?”
He said “Yes.”
I said “Oh.”
The tooter who tooted the flute,
Tried to teuter two tooters to toot.
Said the two to the teuter:
“Is it harder to toot,
or to teuter two tooters to toot?”
I shot the city sherrif.
Sarah Sarah sitting in a Chevrolet.
All day long she sits and shifts.
All day long she shifts and sits.
Sarah Sarah sitting in a Chevrolet. Ole!
There’s no need to nuke unique New York.
(of course)
Peter Piper picked a peck of pickled peppers.
A peck of pickled peppers Peter Piper picked.
If Peter Piper picked a peck of pickled peppers,
Where’s the peck of pickled peppers Peter Piper picked?
One of the coolest URL’s on the Web that doesn’t end with aightdope.com is the 1st International Collection of Tongue Twisters.
– CH