Don't do this

 This is not a big deal; even otherwise good interviewees do this, but don't greet every interview question with "That's an excellent question." There are a few problems with this:

*It sounds insincere.  If the question is a boilerplate "tell me about your dissertation" kind of question, then there is nothing "excellent" about it, and everyone in the room knows it.   

*It is mechanical, as though someone had learned that this is the thing you had to say, every time.

*It sounds a bit immature, professionally speaking, to do this excessively.  

It is ok to acknowledge a question after a job talk is if it truly excellent, but don't say that after every single question. Then you are not distinguishing between great and average and even poor questions, but simply covering your bases. If it is really a perceptive question, then you can something say, "I like that question because it gets to one of the things I had to work out at a turning point in my research." Then, answer the question in a way that makes your compliment to the questioner sincere.  

Asking a good question is also an art form. You can ask someone to elaborate on a point they mentioned in the talk. I'm not fond of questions that try to bring it back to the questioner's own obsessions. (I've been guilty of this, I'm sure.)  Or ones that don't really have much to do with speaker's main point. The point of a question is to let the speaker talk about their stuff off the cuff in an engaging way.