Size? How big or small do you want?
Form factor? A tube mod or a box mod?
Replaceable batteries? Or is a disposable OK?
Do you want no voltage regulation (mechanical mod), single voltage regulated, VV, or VV/VW (APV)?
Cost? As mentioned before cost matters and quality varies with every different mod model at every price level. Remember that you should have two mods capable of equal performance with one as a back-up. When you upgrade to a mod you should always buy two of them, not just one. Things break, get lost, fall into a toilet, etc.