Answer: "Of all the tricks I learned this summer, riding a unicycle is the most difficult".
Explanation: The superlative form of adjectives expresses the highest degree of a quality. Depending on the number of syllables that an adjective has, the superlative adjective can be formed by adding the suffix -est at the end or by adding the word "most" before the adjective. As "difficult" is made up of three syllables, the superlative form of "difficult" must include the word "most" before the adjective. In that way, the superlative form of "difficult" is "most difficult".
