I'll target the plane that represents the biggest threat to me. This is usually the highest con in a furball. But, altitude being equal, I still target the plane that I assume is the most dangerous. I base my assumptions, not on the aircraft itself, but on the type of pilot I typically find flying these planes.
If I ran into the following four planes, I would attack in this order...
YAK - I find that the people who fly these planes are generally above average sticks who got tired of the typical "dweeb" planes.
P-38 - This guy is either the best pilot in this group or the worst. I can tell he's good if he starts to engage. Otherwise, he's just flying a light bomber.
Spit - Skills run the full spectrum here. But, because this plane is the easiest to learn, most of the pilots who fly them aren't much of a challenge.
LA-7 - I encounter the worst pilots in these planes. They're used as a crutch plane and most pilots can't do anything but HO/run. I would say 1 in 20 might have an ounce of skill.
All that being said. Certain planes consistantly give me fits...
KI-84s - When I see one, I'm instantly fearfull.
Temps - The dern things are so fast that I usully don't see them diving on me till it's too late.
109-F - By the time I learn that it's not the K4, he's glued to my six peppering the crap out of me.
Just my 2 pennies.