Hardest tongue twister seen

What is the hardest tongue twister you have ever seen? Humorous ones are also welcome.

I've always liked

Peter Piper picked a peck of pickled peppers. How many pickled peppers did Peter Piper pick?

From the various tongue twister collections you can find (English Tongue Twisters, alpha dictionary English Tongue-Twisters, English club Tongue-Twisters, ...), one of the hardest is the classic:

The sixth sick Sheik's sixth sheep is sick.

But that will obviously vary depending on your accent and region.
As illustrated by this college exercise, it can be a way to improve some specific pronunciation.

There is always the pheasant plucker, it is very difficult indeed in polite society :)

There is a song: http://www.users.globalnet.co.uk/~mentor01/song.htm

She sells sea-shells on the sea-shore. The shells she sells are sea-shells I'm sure.

This one is interesting as it refers to a real person with a fascinating life: Mary Anning, a self-taught fossilist.