Legally I prefer Salt, not too difficult and decent enough money with a reasonably sized truck. Oil also isn't too bad, those would be my choices for legal work.
Drug-wise, weed I would say is the riskiest yet most popular due to its fairly short route from near Agios, to the processor near sand only about 5km away then Kavalato sell. This is the riskiest due to the high traffic in the areas and of course Kavala to sell drugs being the most popular point in the map so be very wary because nobody really minds robbing a drug trafficker and the cops certainly aren't a big fan of it either.
The other drugs aren't too bad and all give good money however again if you want the most profit possible you need to sell in Kavala which can prove difficult without adequate protection.