Depends on how much you want to use it... how much you want to spend too !
In my opinion and based on what I have own (or actually own)...
best bang for the buck... my awards goes to: Badger Universal 360.
It's both, top or bottom feed allowing great flexibility. About 150$ and you're on your way to professionnal finish... hence a little practice.
Best Airbrush period...
This has to be the Iwata Micron.
A powerful tool but must have the skills to operate it and the patience to clean it up if you want it to perform... and beware there's a load of tiny pieces to clean, each smaller than the other.
For an entry airbrush... I would go for almost any Badger airbrushes... they are my working horses really (I get the Iwata out for serious stuff only)... I've tried some Aztec Airbrush and other Testor derived products... try to avoid them... pricy and most unreliable. Plus, a load of tips to interchange.
You may have a Badger 150 for 30$ (the 350 is kind of cool but a hell to clean)... do you have access to a compressor ? ...You must because the can are a pain to deal with !!
Ps... I'm in models for almost 30 years, I shoot on real cars and motorcycle too... but I'm in trains for a couple of weeks... so maybe you'll heard other opinions here... I'm not the one to tell you what is good or not for this hobby, be the judge.