One that puts out a lot of CFM. Most paint guns, HVLP or even suction, require at least 9.5 cfm for uniform performance. Many require up to 16 cfm or more. Regardless of the PSI rating or the HP rating, the CFM is usually the most important factor. Now to get that level of output, you'd probably be looking at a 60 gal to 80 gal twin cylinder compressor at the least. Most smaller compressors won't handle the cfm requirements of automotive paint guns.

That's not to say that you can't do paint work with a smaller compressor, it's just that it requires more effort and time to get a good finish. I know a guy who has painted a number of cars with excellent results using a 30 gal compressor with a 6.5 cfm rating. Because of the smaller tanks these units have to work much harder than those with 60 or 80 or larger tanks. With the larger units there is less time lost waiting for the compressor to catch it's breath. There is also a huge difference between industrial grade compressors and your general grade of compressors. As with many things, quality and value comes with a cost.

I am sure some of the others here, like Hotrodpaint, can give you a far better and more precise answer than I can. They've probably done more painting in one year than I will do in 20 years.