Well, in English, I would say it is:
“The sheik’s sixth sheep’s sick.”
(Try saying that even once quickly to see just how hard it is.)
In German I would say it’s:
“Blaukraut bleibt Blaukraut und Brautkleid bleibt Brautkleid.”
Any suggestions in other languages? I speak fluent Portuguese, but my Brazilian wife hasn’t tought me any tongue twisters yet. (Why does that sound vaguely sexual?)
Black bug’s blood
When I was in college (back in the stone age) in speech class we had to read a poem.
I don’t recall the whole thing but the first line (of about 60) was,
Give me the gift of a grip top sock
Toy Boat. x10
Yeah, I was going to say that one. I can’t say it more than three times without screwing up, unless I say it it very slowly.
But just the other day, for some reason I thought of it and gave it to my son: “I have a challenge for you. I bet you can’t say ‘toy boat’ five times fast.”
Him: “toyboattoyboattoyboattoyboattoyboattoyboattoyboat…”
He had no trouble with it at all.
http://www.geocities.com/Athens/8136/tonguetwisters.html
“Give me the gift of a grip-top sock,
A clip drape shipshape tip top sock.
Not your spinslick slapstick slipshod stock,
But a plastic, elastic grip-top sock.
None of your fantastic slack swap slop
From a slap dash flash cash haberdash shop.
Not a knick knack knitlock knockneed knickerbocker sock
With a mock-shot blob-mottled trick-ticker top clock.
Not a supersheet seersucker ruck sack sock,
Not a spot-speckled frog-freckled cheap sheik’s sock
Off a hodge-podge moss-blotched scotch-botched block.
Nothing slipshod dirp drop flip flop or glip glop
Tip me to a tip top grip top sock.”
From http://www.geocities.com/Broadway/Stage/2203/warmups.html
Mine? “Quixote Quixite quizzed a queerish quizbox”. Just takes forEVER.
Actually, the twister is:
The Sixth Sick Sheik’s Sixth Sheep’s Sick
Think you had trouble with yours? Try that one - it’s the one I would use to warm up for my radio shows when I was a DJ.
Just from reading these few replies my mouth is all bloogly.
[sub]Of course, that’s a word.[/sub]
“Toy boat” comes out “toy boit” after the first couple.
And that sixth sheik stuff…I can hardly say it once.
Biggest tongue twister of all… “I do”
“Unique New York”
Gets me every time… sometimes I can’t even get through the 1st one without saying 'Unique You Nork."
My daughter had one about “soldier’s shoulders”–I wish I could remember it, but it’s probably just as well I can’t…
My wife said I never listen to her. At least I think that’s what she said.
I like “Mrs. Smith’s fish sauce shop”.
When I took theater class in high school, the teacher would have us warm up each day with tongue-twisters. “Unique New York” was one of them, as was “red leather yellow leather.” Now that is a pain in the ass to say.
-Dirty
I can’t even say “Rubber baby buggy bumpers” slow, let alone fast.
It always comes out something like “rubber baggy bubby gumpers”. 
the most pleasant mother pheasant plucker that ever plucked a mother pheasant
My mum’s favourites were:
“good blood, bad blood”
“red lorry, yellow lorry”
“chew chew chew 'till your jaws drop off”
“she sells sea shells by the sea shore”
And the one that earned me a clip over the ears when I came home with it at about the age of 10:
“I am not the pheasant plucker, I’m the pheasant plucker’s mate, and I’m only plucking pheasants 'cos the pheasant plucker’s late”
I can’t blab such blibber blubber!
My tongue isn’t made of rubber!
Seriously my tongue is slooooooowwwww. I can’t even say that sixth sheik one slowly. I have to stop after every word. And that’s why I’m not a rapper.
Red leather yellow leather red leather yellow leather red leather yellow leather.
Not particularly hard, but still…
How many simple shots could Shaq sink if Shaq could sink simple shots?
I’ve read Fox in Socks too, Osiris. 
NOW is your tongue numb?
Games Magazine once ran a tongue-twister contest, inviting readers to compete for the grand prize: their tongue-twister engraved on a silver tray.
The entries included:
Cinnamon Synonym
Ripe white wheat reapers reap white wheat right!
and**
Brock Blake’s black bike’s back brake bracket block broke!
** But the grand prize winner was:
Shep Schwab shopped at Scott’s Schnapps Shop;
One shot of Scott’s Schnapps stopped Schwab’s watch.
A reader later wrote in with another tongue-twister by Dr. Seuss (from Oh Say, Can You Say?:
If you like to eat potato chips
And chew pork chops on clipper ships
I suggest that you chew
A few chips and a chop
At Skipper Zipp’s Clipper Ship
Chip Chop shop.
As for tongue twisters in other languages, the first Book of Lists included one from the African language Xhosa:
Iqaqa laziqikaqika kwaze kwaqhawaka uqhoqhoqha
meaning, “The skunk rolled down and ruptured its larynx.”