Fountain pens are elegant writing instruments that allow users to create their own unique writing style. If you are taking one for your everyday carry kit, then make sure that you are choosing a good brand and a proven model.
An EDC fountain pen must be durable. It should be able to withstand the rigors of traveling which is no small feat. For example, metal pens are preferred over plastic ones because the later can get damaged from frequent use and abuse. You can place the pen in a case as an added precaution.
Metal pens also look more elegant which never hurts when you are writing in public, such as signing bank papers or business documents. Aluminum and brass are the most common materials.
They should be highly portable so small size and light weight are desirable attributes. Lastly, find a nib size that you are comfortable with.