According to tests on American speakers, the most difficult of common English-language tongue-twisters is
The seething sea ceaseth and thus the seething sea sufficeth us.
West Coast Review of Books:
The Ultimate is a well-researched book about where all the best places to ride a roller coaster are, who is the best baseball player, and even the most profitable investment. This book is hot, and Poundstone's enthusiasm for activity and adventure is infectious.