Pirate Transport if you want to be a true Pirate and actually rob people of goods and not 'tax' for credits as its called in a Bomber or something.
Lots of fun too. Fire power (You can take on a Gunboat if your loaded right and pilot it well), armor (w/ Space for Capital Armor if it fancy's you), cargo space, RP a crew, and list goes on.
Gunboats are a better piracy craft than bombers, at least now that bombers have had their antifighter capabilities completely removed. If a squad of fighters comes after you in a bomber, you're dead meat - but if you're in a gunboat, chances are you can destroy them and go about your business.
Transports are somewhat viable as piracy craft now as well, but you have to pick the right ship and the right weapons.
a gunboat, considering they are all around PWNships that cant be effectivly countered. a bomber kills the transport better, but fighters will kill you. Ptrans can die to bombers and fighters, and the omnipresent gunboat swarm. NOT to a single gunboat though, if you have full class 2's. But yeah, go with one of those pwnboats if you want to be able to pirate with near impunity.