The World's most difficult Tongue Twister according to Guinness Book of World Records:

Silly sally swiftly shooed seven silly sheep.
the seven silly sheep silly sally shooed.
sh.......

Denise sees the fleece;
Denise sees the fleas.
At l.......

Can you can a can as a.......

I saw Susie sitting in a shoe shine shop.
Wher.......

How many boards
Could the Mongols h.......

Swan swam over the pond,
Swim swan swim.......