Obviously, both are important. In an engagement involving relatively co-E/co-alt state's with one opponent or perhaps 2 vs. 2 ACM is critical as you will likely be afforded the time and opportunity to execute a traditional merge then manuever into the 'saddle' for a relatively easy shot. In an engagement more complex than that, involving more enemy, the excellent marksmanship ability to quickly dispatch an aircraft with each and every gunnery solution gained no matter how harsh and fleeting is of paramount importance. It is excellent gunnery that can quickly turn an uneven fight into an even one or better when exploited to its maximum potential.
The principle distinction is that excellent gunnery requires only a split second and the harshest of deflection angles to get the kill on your opponent. Good ACM requires significantly more time, especially if the one using the ACM isn't a good shot and/or the opponent enjoys an energy adavantage. This leaves the ACM pilot exposed to the possible addition of more enemy and gunnery passes from his original foe while he seeks the 'saddle', equalized E states and the idiot proof shot.
In the strictest of definitions ACM are largely defensive in nature, unless you are on the deck you cannot kill with ACM alone, yet you can very easily deny the opponent a kill on you for a protracted period, possibly even indefinately. Gunnery on the other hand is offense, pure and simple. An excellent marksman need only have a solution for a fraction of a second and it's over, with an initial energy advantage he will almost always be afforded at least one such opportunity no matter how good you are at ACM's. The best defense is a good offense, great gunnery means faster kills which in turn means you are less likely to get cherry picked, bounced, BnZ'd, gang-banged, etc.
Of course there are those blessed with both skill sets in abundance, some of whom have been mentioned in this thread. Naturally, that is the ideal.
Excellent gunnery is the ability to kill quickly and proficiently from almost any angle at any reasonable distance with any weapon system. Good ACM is the ability to manuever in such a way as to deny your opponent a gunnery solution on you and hopefully produce one for yourself eventually, relative E states not-withstanding. All of us possess ability in both of these areas to some extent. Recognizing your strengths and maximizing them through engagement and plane choice is the difference. Also, recognize your weaknesses and work on strengthening them and minimize their negative impact by engaging in such a way and with a plane that does not further exacerbate those weaknesses.
Anecdotal history from WWII gives thousands of examples of this. Each successfull pilot had a 'killing blow' as it were, a method of attack and fighting that afforded them, personally, the greatest chance of success with their particular plane and their individual aptitudes and God-given talents. Aces High does a great job of recreating this 'feel'. As a direct result of this, almost all of us become 'niche' flyers to some extent, choosing to approach air combat in a way most conducive to our individual success, talents and personal enjoyment of the game.
Just as in AH there were the Zeke-type pilots who used the high and low yo yo's up close and personal to great effect. There were the 109 pilots who used altitude and the devastating 20mm laden BnZ pass. There were the P47/P51/F4U pilots who used power dives and zooming spiral climbs to energy fight the opponent into submission. It is the amalgam of all of these seemingly disparate methods, aptitudes and proclivities that makes WWII air combat history and AH so very interesting, paradoxically complex and enthralling.
Personally, to answer the original poster. I would have to say gunnery is the most critical in the MA. I only say this because, as mentioned above, ACM's alone are only most crucial to success (defined as destroying your adversary without yourself being destroyed) in a 1 vs. 1 or 2 vs. 2 co-e/co-alt similiar plane-type situation. Those situations are exceedingly rare in the very finite confines of the populous MA. In a crowd, where there is not likely sufficient time to safely work angles on a single opponent for a good gunnery solution or the aircraft in question have widely varying performance characteristics, marksmanship ability is master.
Zazen
