According to researchers (I don't know who) this is apparently the hardest tongue twister ever:
Pad boy poured card pulled cod. (What does that mean?)
and this is another good one:
Suzie sits in a shoeshine shop.
where she sits, she shines.
where she shines, she sits. (YIKES)
which is reminiscent of my personal downfall about a hundred years ago:
i split a sheet. A sheet I split.
And on this split of sheet I sit.