Remember, tiers are relative. What makes a top tier in Marvel 3 is different from what makes a top tier in KOF or SF. Tiers are a generalization anyways of how many good and bad match ups a particular character or team has against other characters or teams, it doesn’t speak about the actual tools each has.
That said, I’d say to help elevate one’s game, I’d say pick a character that embodies the fundamentals of the said game. If it were KOF in general like 98og or 02og, I’d recommend Iori. Iori is top tier in 98, but only high tier in 02, but he still teaches KOF fundamentals and has the good dirty shit (and plays different archetypes like zoning, grappling, and high/low offense all in one character) to get you to win.
If it were Third Strike, I’d recommend Ken. Sure he’s not as “easy” or has as good buttons as Chun Li and isn’t as top tier as her, but he teaches the fundamentals of whiff punishing with decent buttons, hit confirms off lights, shimmy stuff, cl.MP > tick throw or cr.MP frame trap into hit confirm super, has a meterless reversal, and stuff like that. What makes Ken a good fundamental character in Third Strike is different from what makes Iori a good fundamental character (although some aspects may overlap, but not all.) I heard for (and it might be hearsay) Jacky was the go to character in VF4 to teach you the fundamentals of the system while it was Wolf in VF5 that taught you the fundamentals of that game even if VF4 and VF5 are somewhat similar in nature.
That said, I think to be a better player also requires the player to be a better student and researcher. Go and ask better players their thoughts, but don’t take them as gospel though; respectfully and mindfully consider what they have to say in earnest. Try to find multiple sources of information and try to gauge who to listen to when there is contradictory information, and this is where you use logical thinking and inferences.
In a quick general statement though to reaffirm, I’d say pick a character who could do a little bit of everything well within the context of their own game. If you have a top tier who could do that but even better, then pick that character. If none of the top tiers seem to do that, then the game is balanced in a weird way where it’s more RPS of character archetypes at high level play and pick the archetype you’d really like to learn or enjoy playing as; but remember, this is only if there is huge skewing of character choices in competitive play. Or rather, maybe the skewed top tiers of that game is indicative of what the “actual” fundamentals of that game are.
If there is any other way to improve as a player, the key things to look out for is being able to defend against options (as well as blocking better), being able to whiff punish, being able to convert as many situations where you hit into damage, and how to convert your hits into very advantageous situations that eliminate variance and/or risk.
Iori is an example of a character in KOF where I can whiff punish, convert into good meterless damage, have metered and meterless combos go into hard knockdown that gives me a safe hop mix up (advantageous situations that eliminates variance & risk), and has a reversal command throw and DP in KOF98 so he gives me options to defend where other characters like Lucky Glauber wouldn’t be able to shake someone off. Of course, still work on not overly relying on DP and command throw and just block properly, but having the option available vs. not having it is also key to a character pick and allows you as a player to learn when to use those tools properly and work on self discipline to not use it even when you do have it. Iori’s tools, although strong, aren’t one dimensional either so it’s not like picking a top tier with one really strong button without thought wins you the game (lol Rose Alpha 2). So he’s top tier, teaches fundamentals well, and does everything I want to have an effective game plan to win as mentioned in the beginning of this paragraph.