1.Most often used in English the letter E, as least likely to - Q.
2.
The most ambiguous words in the English language - set. It has 44 basic values for the verb, the noun for 17 values, the values of 7 for an adjective and a few hundreds of different options. So when translating this insidious words need to be especially careful.
3.The most difficult tongue twister in the English language: The sixth sick sheik's sixth sheep's sick.