Despite the fact that we may have worked for the most incompetent/evil/unorganized people in the world (in our opinion) we never should ever say that or bad mouth them in an interview. For all you know, this company could have them as their largest customer so you effectively eliminated any chance of employment. Some more correct responses are as follows:
I found myself bored with the work and looking for more challenges. I am an excellent employee and I didn't want my unhappiness to have any impact on the job I was doing for my employer.
There isn't room for growth with my current employer and I'm ready to move on to a new challenge.
I'm looking for a bigger challenge and to grow my career and I couldn't job hunt part time while working. It didn't seem ethical to use my former employer's time.
I was laid-off from my last position when our department was eliminated due to corporate restructuring.
I'm relocating to this area due to family circumstances and left my previous position in order to make the move.
After several years in my last position, I'm looking for an company where I can contribute and grow in a team-oriented environment.
I am interested in a new challenge and an opportunity to use my technical skills and experience in a different capacity than I have in the past.
I recently received my degree and I want to utilize my educational background in my next position.
I am interested in a job with more responsibility, and I am very ready for a new challenge.
I left my last position in order to spend more time with my family. Circumstances have changed and I'm more than ready for full-time employment again.
I am seeking a position with a stable company with room for growth and opportunity for advancement.
This position seemed like an excellent match for my skills and experience and I am not able to fully utilize them in my present job.
The company was cutting back and, unfortunately, my job was one of those eliminated.