Cobra III is the best smuggling ship. I run smuggling missions from Maia. Making 5-10 million per run with 32 cargo. I have A class everything for heat management and a class 4 fuel scoop. No weapons, no shield, an advanced discovery scanner and two hull re-inforcement with mirror surface armour. Not been scanned yet.
But Frontier has nerfed smuggling recently. You'll get one good run per day, but the rest is picky at best. Still, I'd much prefer to run 4 million than trade. Yeah, trading and mining can better smuggling now, but at the cost of fun and excitement? I really hope the 2.1 mission system is overhauled in it's entirety.