I've been decorating cakes for the past 12 years and have just now started decorating cookies. I've seen some amazing airbrushed cookies. What's the best airbrush for a beginner?

I guess it depends upon how detailed the work you wish to do.  My airbrush is a relatively inexpensive Master brand, actually two airbrushes and a compressor in the set.  It does a wonderful job for tinting and highlighting cakes, but I've never been able to figure out how to do fine, detail work.  It very well could be possible and I just have not yet learned to use it properly.

I've also heard good things about the Dinky Doodle brand, which is small enough to be easily portable.

Thank you. I think I would use it more for highlighting and shading. I've also seen a lot of cookie stencils that look like they would make decorating faster and easier.

It’s been far too many years since I’ve had my hands on an airbrush to be of any real intelligent help ;) 

For those purposes, I think most brands would work.  My little cheapie would.