The one that's the biggest threat. Assuming 3 planes all about the same distance, spread out around me, all around the same potential to turn on me any second, I'll pick on a spit16 before the P-51 and 109E-4.
Because if I don't take that spit16 out and I give him a single inch anywhere, he'll kill me or make it so that I get in a position (from trying to evade him) that all 3 cons are now killing me.
Barring any immediate threats, the closest available target that doesn't have 4+ already chasing him.