Every aircraft seems stable in Aces High in the plainest meaning of "stable" -- that if you are flying along straight and level, the plane stays straight and level.
If by "stable" folks mean how wobbly the aircraft is to control inputs, though, in the sense of control theory, you are talking about step response, which is this sort of thing (where the square wave is your input, and the oscillatory graphs are the response or output).
This is what happens when you are flying along at a given position of the stick, then move the stick to a new position. Does the nose oscillate in response to a step change to pullback on the stick? Does the nose oscillate back and forth if you initiate or stop a roll? Etc.
In this definition of stability, although I haven't done any tests and don't thoroughly remember nuances of the feel of many AH aircraft, what comes most to my mind is the FW 190. 190's seem very crisp in control response and seem to have low oscillation. The F6F, for example, to me seems to have a great deal more.
