So far, my take (after finishing maybe 10 times on hard) is the two top choices are Bolter Fighters and Plasma bombers.
Bolter fighters are the best at blocking enemy fire due to their larger size, and faster respawn, they also do decent damage.
But if you want damage dealers, Plasma Bombers are it.
Lot better than missile bombers, in a tough fight missiles are often blocked by enemy fire while plasma just goes through.
Theres also more plasma bombers than missile bombers, so.. easy choice.
Bolter and laser drones are in between. Decent defense, decent offense.
Personally I'd rather focus on one or the other.
Repair drones are also a viable choice, often meaning you come out on top in a prolonged fight.
Shield drones the same, only less so. Good to have if you have lots of fighters and like to end up with lots of scrap.