Best way to make money IMO:
Most Dangerous but highest reward: Join a small gang that doesn't bother much with cartels, ninja-cap a cartel when there are few gangs on and take the money for yourself.
Sort of Dangerous but good reward: Run Scotch because it isn't very frequented.
Legal/Safe : Wreck excavating, although be aware you will pick up Coke which you can sell but dont get searched. Dont forget, your boat has a trunk.
I won't bother recommending HEMMT runs for diamonds, you're just asking for too much trouble. And never pull out a HEMMT in kavala, just feel like you should know...