most difficult thing in vs most difficult thing of

Examples:

• The most difficult thing in playing the piano is coordinating both hands.
• The most difficult thing in cooking is getting the timing right.
• The most difficult thing in solving this puzzle is finding the starting point.
• The most difficult thing in writing a novel is creating believable characters.
• The most difficult thing in studying for the exam is staying focused.

Alternatives:

• most challenging thing in
• hardest thing in
• toughest thing in
• most complex thing in
• most demanding thing in

Examples:

• The most difficult thing of all is to admit when you're wrong.
• The most difficult thing of the day is waking up early.
• The most difficult thing of the job is dealing with difficult customers.
• The most difficult thing of being a parent is balancing work and family life.
• The most difficult thing of all is to let go of the past.

Alternatives:

• most challenging thing of
• hardest thing of
• toughest thing of
• most complex thing of
• most demanding thing of