Fighter Rank in and of itself a poor measuring tool because by its very nature, it's relativistic. However, the components that comprise fighter rank are absolutes and do have some comparative relevance. That is to say Hit %, K/T, K/D and K/S do indicate the prescence of aptitudes in specific areas that are key components of aggregate talent in fighters. Of course there is no measure for one key component and that is raw flying ability, that is unquantifiable, very subjective and situational.
So, to define an Ace, you really have to qualify that with, Ace of what..? Very few people, I can think of only 2 or 3 I've ever known in 17 years of this, can pick up any plane out of the hanger, fly it in any style and be equally successful in it. By that I mean there are Aces, there are TnB Aces, there are E fighting Aces, there are snipers, there are those with great flying skill but awefull aim, or people with great aim and flying ability but piss poor SA etc.
Humans will tend to, after sufficient experience, cultivate the self-awareness to figure out what aptitudes and weaknesses they have and fly in a fashion that maximizes those aptitudes while at the same time minimizing their weaknesses. In my personal opinion those are the Aces.
If you want a pure definition, an Ace is a pilot who flies in such a way as to maximize the effectiveness in combat of both his own aptitudes and his chosen plane's strengths while at the same time minimizing his own weaknesses and the strengths of his opponent and his aircraft. Someone who can consistantly accomplish that flight after flight, encounter after encounter is an unqualified Ace regardless of what particular area(s) he excels.
Naturally, in the game there are certain activities that can statistically perpetuate the illusion of Ace-dom without actually possessing the reality of it, vulching, 262 and 163 buff farming missions and more clandestine methods involving other accounts for example. This is the biggest reason Rank in and of itself or any sub-stat of which looked at in isolation or out of context is not particularly meaningfull. But, blatant padding notwithstanding the core attributes that comprise fighter rank, not the fighter rank itself are, if nothing else, fairly pure indicators of pilot aptitude in a given area. Taken as a sum total they do tell a story and combined with the correct plane to accentuate those proclivities do end up becoming, to a certain degree, a measure of that person's overall effectiveness in the MA with a fighter.
Zazen