Lists should be based mainly on theory.
Results should be taken into consideration as well, but not as a major factor, because most characters are underrepresented.
I strongly agree with the wording here: "mostly."
The tier list should be an accurate reflection of the application of theoretical traits, which is where the "Proven capability" I've mentioned comes in. To elaborate one what I mean by that if I can, it's taking what is there in theory, and showing what that can do when applied. This means looking at the best players each character has to offer and comparing that "best of a character" to the "best of the other characters."
This sounds like common sense, like well duh, of course tier lists should reflect the top level of play, but it astounds me how little contemplation there is from some people (Not firing shots at anyone in particular) about what constitutes "top level." The Super Theory Bros. side would contend that top level is at frame perfection, where Fox can shinegrab, nairshine drillshine, infinite waveshine, and all sorts of other mindblowing offensive things with absolute precision to his heart's content. The other, even more ignorant side, the "Results reflect all" side, is that the top level of play is who is getting top 8 or top 4 or just who's winning tournaments, which is just frustratingly inaccurate and is the cause for why no one knows were the **** to place low tier characters.
The third option, not even the middle path because **** Buddhism Daoism is the best, is examining the top as being the highest thus-far proven application of characters. It isn't just looking at whoever is considered the number 1 player for every character, but for many characters, especially mid and low-tier ones, that does have to be the case as that is the only reflection of the understanding of those characters our community as a whole has. Even if it does mean Marth is doing more poorly because no one is good enough with him, we have a tier list that is much more willing to change than those of other games simply because of the complexity, variability, and ever-growing nature of our metagame, so it's fine if Marth has to sit back a few spots until Marths can be like "hey **** all y'all, check this Marth **** out and tell me Falcon and IC's are better than me anymore."