The best fuel to coin ratio mission is 1v1 OD6, not only that but it also gives good xp. It also only has 3 enemies, so you can do it fastly. I remember that a 3v3 mission is good for coin grinding too, but your mech isn't ready for it.
(example;tanks, fighter jets, melee weapon {like the karambit}, EMP, heat hugger, etc)
Can be from any year. So you can go with sticks and stones from year ooga booga. I'll start with the flying AC-130, it's not the best, but it's not the worst. Quite reliable when air support is needed.
My honorable mention has to be the [REDACTED] from the year [REDACTED], it's such a great weapon to knock people out.