It's not specified in the question, so I'll assume that it's a fixed-wing SEL airplane (aircraft is anything which flies, including balloons, rotarywing, etc...). I'm also assuming you can't do exotic things like change the engine compression ratio.
I will also assume that efficiency in this context is only energy consumed per unit height. If we want to define efficiency as energy consumed per unit distance traveled, then the problem moves in a different direction and becomes much more complicated.
Please understand that these are first-order effects, but second-order effects can lead to slight modifications. For instance, maximal efficiency might come at a slightly faster/slower airspeed in order to allow for a slightly faster/slower engine RPM.
Also, in general, fuel is not the important consideration for takeoff. Safety is. It's better to burn a few mL more of fuel and keep the airplane from disaster (which would presumably result in wasting a lot more energy).
Mixture (with important caveat at the end)
The mixture is the biggest control on efficiency. In order to achieve maximal efficiency, you need to run as lean as possible. Preferably lean-of-peak, if this can be done. We're talking 20% savings or more here.
Caveat
If you're not taking off in the mountains, don't ever try this if you value your airplane engine's life, and as a consequence your own. The engine needs the enhanced cooling from the rich fuel/air mixture. Only at high density altitudes can/should the engine be run at full power at extremely lean settings.
Throttle
The throttle should be fully open, which will provide the least restrictions to the engine breathing.
RPM
If you have a constant-speed prop, then run it as slowly as possible. This gives maximal time for the burning fuel energy to be extracted, as well as minimizing the number of engine cycles and friction.
Gas
Use the lowest octane gas you can, as this has more energy content than higher octane. Please note that lower octane increases the propensity of the engine to knock/detonate.
Airspeed
Fly the plane at Vy, which is best climb airspeed. This is the airframe's most efficient operating point.
Flaps
In general, don't use them. They increase drag more than the increase lift. That's why we like them on landing because we can come in at a steeper angle of attack.
If this seems non-obvious, we can recognize that higher aspect-ratio wings are more efficient, and flaps will decrease the aspect-ratio so the wing is more optimal without them.