There are several the Casino and Galactic missions are among the best.
The casino is by far the best because it has a more often than not positive payout and you can save and reload before entering if you end up losing more than you are willing to before hitting the winning streak.
Galactic mission how these work has changed several times but usually the rewards are a nice sum of money and materials. Currently I am unsure of how they work exactly as that changed a lot since I last messed with them.