Best experience for the energy spent.

[FluffNStuff]

There is another aspect you may be missing, which is to min max energy/level increase. If you are close to leveling, do the minimum xp missions to get you one xp under what you need to level. Then do the biggest mission you can find. Optimal results would be with a multi hour mission ready to cash in. Remember, you do not get the reward till it is cashed in, enabling you to save up the pricey one til you are ready to level.

Best bet is an action board to level because leveling with a mission a lot of times gives you energy you lose because of the flip.