What the right answer is can very very situational.
First they want to know you aren't going to bad-mouth them when you move on from that position. So the first rule is no matter how annoyed you are at your current employer (and if you aren't annoyed at something you probably aren't looking for another job), do not say anything bad about them. So your phrasing is important when answering this question. "I want to move on to something more challenging" is far better in this case than "Company XYZ won't move me to a more interesting project." in the second case you come across as a disgruntled employee who will always be wanting some special treatment from them. The second answer is better than "Company XYZ is a loser company and I hate them." but not by much.
Now the other thing they are looking for is whether the work they intend to give you will be interesting enough to keep you there. So if the job you are interviewing for does essentially the same thing as you are currently doing, then "I want to move on to something more challenging" is the wrong answer. However, if it true that you really want something more challenging and the job doesn't offer that, then it is best for both sides that you find out before you take the job. If the job offers something new that you don't know (say a SQL Server dba who would now get to manage both SQL Server and Oracle database), then a better answer refers to the thing you want to learn. "I'm interested in learning Oracle." then becomes a better answer (assuming you aren' trying to pretend to knowldge you don't have.)
Generally if you can make the answer about something interesting that the description of this job will bring you or why you have always wanted to work for this company or something general about new challenges, you are fine. If you are working under a contract that is about to expire or have been laid off, that too is fine. Saying you need more money can be fine (as long as you don't sound super greedy - your application says you make 30K now and you want to get 150 K for instance). Given the current economic climate, saying that you want to change to a more stable industry can be good as long as you are currently in a bad place like a bank and are looking at something that will be considered more stable(probably not a good answer if you are in the real estate industry and are interviewing at a bank).
Sometimes being very honest with this question can turn off a potential employer. On the other hand, that often means that would have a been a bad fit for you. I once quit a job because of the excessive overtime (And I do mean seriously excessive I was working more than one 18 hour day a week and the shortest day I working the last four months before I quit was 11 hours). I honestly told people that was why I had quit without having another job. There were a couple of places that turned me down, but the place that hired me didn't expect that kind of hours. Now note I didn't say the company was wrong to have that expectation of hours, just that I personally couldn't physically handle that level of hours. And the real benefit was that they only asked me to work extra hours very occasionally knowing that I didn't want to do nothing but work in my life.