what is the best airbrush to buy???

Just to give you a warning, this is quite a subjective question.  Iwata users love iwatas.  Badger users love badgers.  Aztek users love...well they use Azteks.  Paint cup is usually by preference.  Siphon feeds hold more but require a higher air pressure to spray.  You can swirl siphon fed airbrushes when using metallic paints easily.  Gravity fed airbrushes operate at a lower pressure and hold less paint (which can be a plus when you're doing a small amount of spraying or a minus when painting a very big model).  Sometimes the gravity feed cup can get in the way when painting, though.  Ill let others with more experience give their views.

                                                                    -graham
 

There is no such thing as a "Best" airbrush 

http://www.craigcentral.com/models/ab.asp

MusicCity is right, there is no such thing as a "best" airbrush, and the link he's posted is the one I would have if I had arrived here first!

I solved the siphon vs. gravity problem by getting a Badger 150 & 100LG. They are the same airbrush, with the only exception being the method of paint delivery. 

Some people will say that a single action airbrush is easier for a newbie to use, but I think that's a subjective answer. I chose dual action and never bothered with going the single action route. Though I am considering getting the 200G, which is the single action version of the 100G (1/16 oz. cup), on the theory that too many tools is better than not enough. 

The trick will be practicing. Any decent airbrush will only be as good as the hand that controls it.  Keep it clean, experiment with thinning paint, air pressure settings- and practice. Using a plastic milk carton is good. I actually built a simple 1/72 scale kit (I build aircraft) and painted it more than a few times as a practice/experimental "canvas".

I'll add to the chorus of "there's no 'best' airbrush."  For model building, an inexpensive single action airbrush like the $50 Paasche H will give excellent results and do almost anything required.  There is a common belief that an expensive dula action unit is needed to do narrow lines and free-hand camo, but for tanks and aircraft 1/72 and larger, the Paasche H is quite suitable.  Needless to say, anything higher end, (dual action Badger, Iwata, etc.) will also do a good job.

The main thing is the skill (which really translates to experience) of the person using it.

As everyone else has said there isn't an airbrush that can be called the best. Though there are some that are harder to work with. Like the Testers Aztec. I had one of these when I started and got some decent results but I found it very inconsistent. But I have seen some people get results with it that I could only dream of. For me the Iwata line is a great line. I have the CH-CS and I have been very happy with it.

It all comes down to personal perfence and skill. If you are starting out start with a simple unit to get some experince then when you feel comfortable and have the money to invest in something more advanced then take the plunge and give it a try. Look for art clinics or hobby shops that might give lessons so you can try out different models of airbrushes if possible. If not then read as much as you can, there is alot of info. about airbrushes right here just do a search.

Main thing is to have fun and find what you are confortable with using.