Biggest you can afford if you're using it for a gun. Oilfilled, the ones that typically look like they have an engine on top of the tank, are quieter, and I think tend to be more powerful and last longer, but they are pricier. Also, belt drive is quieter then direct drive: With belt drive the motor turns slower by driving the pump with a belt, and the motor needs to turn alot faster to drive it directly. Direct drive kinda scream.
With CFM, you need to match that with your gun. If you want to use a gun that requires 6CFM, you should get a compressor that can put out at least 6CFM. If you go smaller with your compressor, like I do for instance, I use a gravity feed that requires around 6.5CFM with a compressor that can put out around 3.7CFM, your compressor will kick in alot more often, and have a shorter life.
I'm not sure if HP matters as much, it's mostly CFM and Gallonage.
Another thing, I've heard that CFM is not that same as SCFM. SCFM is supposed to be a more correct, but smaller number, so companies sometimes list in CFM instead of SCFM, but I might be wrong.