While the above works as a general guideline, most folks rank heroes based on their own experience with them, whether owning them or fighting against them.
It’s obviously not a perfect system and much depends on personal preference and team synergy, as well as what you’re using a hero for—raid attacking or defense, tanking or flanking, events, quests, et al. Our experience in raids probably carries the most weight, as its the central ranking system in the game.
To that effect, since most of us (who don’t have unlimited pockets) don’t own every hero, our rankings are based centrally on who we find to be the biggest pains in the ■■■ when raiding.
Beta testers have a decent enough handle on how newer heroes play and are typically more experienced with older heroes as well, but even they (we) aren’t wholly reliable.
I’m sure none of this is all that helpful, but it’s worth recognizing that these rankings, while fun and helpful in terms of establishing countervailing opinion, are incredibly subjective and not necessarily the result of critical or definitive research.