Which is the best mission to farm funds/money?

[BigFatJuicyMonkeys]

Either directly from the quest or from selling the mats.

[DeadByLag]

Use Christa; she gets bonus cash.

However in the long run, your problem will be mats, not cash. As you grind through the missions, you have to sell off other low-grade surplus mats before they hit max limit (which is 999 I believe), and bonus cash will flow in for the regiment as you complete more of the gallery and muster roll.

If you have serious intent on improving equipment via fortification (which helps to make True Attack mode easier), you're in for one hell of an ultra tedious grind on certain missions (for statements). Read the fortification guide

https://steamcommunity.com/sharedfiles/filedetails/?id=1172044113

The materials database https://aot-wof.herokuapp.com/en is not 100% accurate but useful enough. Use Sasha to get bonus mats.

[BigFatJuicyMonkeys]

Yeah, but you can buy mats. If you can make a lot of money, it might be faster to just farm the money and buy what you need. Apparently the DLC has a map for a lot of money but I don't want to buy it if I can help it though.