This question scares people but in reality you should be hoping you get asked this question for the very reason that it is open ended, which allows you to define yourself for them.
In an interview, you are selling a product - you - to a potential buyer. The buyer usually knows what they want - you need to find out as best you can what that is and then position yourself in a way that makes you the fulfillment of that need.
You must have your answer to this question memorized and well-practiced - no hesitation or fumbling for the right word, but watch for non-verbal cues to judge your effectiveness - are they confused, bored, etc. Ask if they want you to elaborate on anything - but only if you are prepared to do so.
You should focus on 3 specific skills that you have that they need. One innate skill, one learned skill that is generic enough to use anywhere, and one specialized learned skill that applies specifically to the interview situation.
Most interviewers want to hear about accomplishments and problem-solving, and they love specifics - you created a database with 420,000 records, or increased sales at the book store by 17%, or you've designed six projects in some specific CAD software. Give them some numbers.
Use short, punchy sentences. Repeat your skills - your brand - several times during this answer and throughout the interview. You want to be memorable. When they think of you later, those 3 skills should come to mind.
Also, don't say anything negative about anyone or anything ever - be positive. And it is OK to say outright "I want this job." That could be an influential factor, even subconsciously.