...Or at the very least the best one I've tried thus to far.
Reason #1. THE VIEWING SYSTEM! 
I can't stress this enough. Every time I try to play another combat sim besides AHII, I find myself missing the viewing system. This is the only sim I've ever tried that allows you to use buttons on your stick to track the bandit with anything resembling the ease of simply moving your head to track moving objects in the real world. While simultaneosly being able to maintain situational awareness in a multiple-bandit situation, the Achille's heel of all sorts of "padlock" viewing modes.
No matter how good in other respects different flight sims I've tried are, you always find yourself missing the the AHII viewing system and not being able to proceed about the business of maneuvering the airplane nearly as effeciently.
Reason #2. Readable instruments. You should be able to check your instruments with a glance at the forward view of your cockpit. AHII is almost unique amongst combat flight sims in actually being able to do this.
Speaking of instruments, the highly visible G-meter in AHII is a God-send. Even though most WWII airplanes didn't have them, it is entirely appropriate to mount a G indicator in any sim cockpit, to make up for the lack of physical feeling of G forces. Same goes for the audible angle of attack indicator, or stall-horn we have. Without the subtly input of control feel, entirely appropriate to model it into a sim.
