This is another one of those questions with no answer... like "What's the best flavor of ice cream" or "What's the best muscle car?"
Since everyone will have a different opinion as to which airbrush is the "best," and there really is no one "best" airbrush, I won't name any specific brand. But I will say that you should get the best airbrush you can afford right up front, and that airbrush should be a double-action model. Here's why...
A cheap single action airbrush (single action being the type where you can't really adjust the ratio of air to paint... you pull the trigger and it sprays, like a spray can) can give you a nice paint job, but it's limited in what it can do.
A double-action airbrush lets you control the air and the paint independently. You pull back on the trigger to spray air and you push down to release the paint. A double-action model lets you do a lot more, because you have total control over the spray action and how much paint the airbrush is putting out. You might not need the capabilities of a double-action airbrush right away... but as you get more into airbrushing, you're going to want that extra control... so better to buy one in the first place.
A cheap "starter" airbrush costs less up front, but the odds are you'll soon outgrow it and want a better one... so why not buy the good one up front? It costs less than buying a "starter" model and a better one down the road.
A good airbrush, taken care of, will last literally a lifetime. You may have to replace a seal or two down the road one day, but the airbrush itself will outlast you.