Often than not, this question is asked a lot during the interview, "What is the most challenging project you've done in the past?"
This type of questions is to measure how you would go about solving problems. A typical answer involves the following steps:
-a description of the project
-what are the problems
-how they are solved and what's your role and contribution to it
To answer this question well, you need to prepare it beforehand. Below are some sample scenarios that could pose challenging to an IT/software related career:
1. Technical Career Challenge
--Assigned a task where it has to be done in a given technology you don't know. How you got it done with effort and also learned the new technology.
--Under extreme time pressure. After putting in much effort, finally solved the problem.
--A bug was holding back the release. After much hassle, finally found a good solution.
--Team members don't give a helping hand. After effectively communicating with the relevant member, got the problem solved.
--Inherited a crappy old system, how you migrated it to the new platform and without issues
2. Marketing Career Challenge
--With very tight budget, how you lowered the cost and boost profit?
--Customer was difficult to work with. How you effectively communicated with the customer and gained his/her trust.
--It was hard to measure the effectiveness of a marketing channel. How you figured out the correct tracking method.
Also, sometimes you don't need to give successful experience. Even a failed attempt at a challenging project could demonstrate your ability and perseverance. The main point is to show case how you did it and what you learned from it.
To better prepare this interview question, there is a good book that could help. It is called 101 Great Answers to the Toughest Interview Questions
. If you are well prepared with these 101 questions (ie apply yourself to these questions and see how you can answer them), usually no problem with any behavior questions.