I believe you can get missions close to a million on the Isis, capspam ftw.
I think the missions might have bigger rewards deeper into Nomad space, but they'll be way too risky to solo.
Ruiz Base, Omicron Beta. Good place to do missions in a fighter (Outcast aligned, obviously). You'll never fight more than Eagles, and the missions average in the 600k area but get all the way up to almost a million credits- and at that point the GMG you're fighting will drop roughly 15k each in pilots and credit cards.
The anti-Hessian missions in Omega-41 are also pretty good for fighters to do, but the radiation makes it painful for bombers (note: I am aware that Beta has radiation as well, but it's barely noticeable). Corsair GBs can make a lot of money in 41.
virtually every mission in gallia goes from around 300 000 to 890 000
Also worth noting that those missions are some of the only times I have been attacked by NPC bombers
For GMG go to Okinawa and take mission from Fujisawa. I've done missions just shy of 900k over there. Never seen higher than fighters but I should mention that the Hogosha NPCs soak up a lot of damage.