you start the engine which the aircraft flight manual tells you to start first
you should start the one that burdens the ship's systems the least.
big jets (787 aside) use a pneumatic starter so it usually does n't matter, but it should be the engine away from cargo doors or other ground facilities
IF it is an electrical starter system, you start the one with the shortest cable to the battery as it gets slightly more power that way (B17).
wing loading is good, if you don't load a wing you don't fly