Like many other things, it depends on what you want to do with it.
To start off with, I'm going to assume we are looking at something to paint models with and not painting automobiles or houses.

So, I'm going to focus on the smaller airbrushes.
There are many different brands and each person is going to have their preferences based on what they and their close friends have used. There are many good brands out there. And all of those brands have good and lower quality airbrushes.
The 2 most common to modelers are going to be Pasche and Badger.
Then there is single stage and dual stage to consider. Single stage you have to adjust the needle before you start painting with it and the needle stays there during the painting process. The position of the needle is what determines the "pattern" of the spray. This is what people consider the airbrush to start with because it is easy to learn due to not having as many things to vary to get a good finish. The single stage airbrush is also easier to clean. This is also what people will use when painting to replace "rattle can" paint. There are still things to learn like thinning the paint to the right consistency to get a good finish is probably what most struggle with. Dual stage you push the button to spray and then move the button forward and back to adjust the position of the needle to adjust the pattern as you paint. This gives you flexibility in adjusting the spray pattern as you paint. These are considerably more challenging to learn to use well.
The next thing to consider is if you want top feed or bottom feed. Top feed is for painting with small amounts of paint that goes in a cup on the top of the airbrush. The bottom feed uses a small bottle slung under the airbrush that can hold more paint. Obviously, the bottom feed brush holding more paint can paint larger areas without having to stop and refill the airbrush reservoir.
Things to also consider are the compressor and tank to use, an air regulator and a moisture separator, or you can get little cans or propellant to use but they don't last long at all.
Sheesh ! Making me type so much first thing in the morning.

(I'm just kidding)
I hope that helps !
Jeff