Most difficult question was an OOP application architecture question posed as a client request. As you solved each part to the satisfaction of the interviewers, the request would morph to put stress on your architecture requiring you to rethink the best solution.
Sigiloso
You really can't fake this one, having been the interviewer here as well, I've seen many different types of answers and all are acceptable so long as you are fully addressing the needs of the application and understand both the advantages and weaknesses of what you are implementing.