I have no experience in this area, so you can pretty much ignore any advice that follows.

I think I would be very honest about this question. I would tell the interviewer that my current job bores me and I don't feel intellectually challenge, which is important to me. If the interviewer doesn't understand why this is a problem then this company might not be one I want to work for anyways.

Interview is all about honesty. I think there are 2 things you could say. The first reason is that you have been in your current job for a long time and you have learned a lot and became good at it. But it is not giving you any more challenge and growth that you are looking for professionally. Another reason you should think about is why this company is a better company than your current one. There is got to be a reason why this is a better fit for you professionally and personally. It could be what the company stand for, what they sell, or the technology you will be using, you name it.

You just answered it. That's a perfectly fine, honest answer. The interviewer might follow up with a question about trying to get another position within the same company so be prepared to answer that.

That is true, who are they to look down on me for giving an honest answer. I already tried that many times haha, bottom line was I didn't have an engineering degree!

Tell them the truth.

"My current work is boring and stale maintaining intranet web applications. Been doing this for 5 years. I believe I've exhausted all opportunity for growth at current job. I'm looking for a change and a challenge."

Agreed. You definitely answered your own question. Just tell them the truth. There is nothing to be ashamed of in that answer.

"Ready for something new" is the standard and boring, but fair answer to a totally unfair question.

Why is it unfair question? Other than they may not have a current job.