Guess I'll display my Tier List since I've mentioned it multiple times. For Pre-Genesis: Based on a combination of results/theoryworking/etc
Top Tier: For characters that clearly are generally considered to have the most consistent and best results, have the best tools compared to the rest of the meta, etc. Applies to Pikachu and Kirby in 64, Fox and Falco in Melee, and Ice Climbers in Brawl. Meta Knight in Brawl and similar occurrences would most likely require another exclusive Tier to emphasize dominance. E.g., SS Tier, God Tier, etc.
High Tier: For characters that have fantastic tools, but might have slight weaknesses or flaws that keep them from being dominant. They often can clash with the best. See; Marth, Sheik, and Floaties in Melee, Fox and Falcon in 64, and Diddy and Olimar in Brawl.
Upper Tier (Upper Mid): For characters with some clear advantages but noteworthy disadvantages that allow them to compete at a higher level but often struggle to win serious competitions. See; Ice Climbers , Falcon, Samus, and Pikachu in Melee.
Mid-Tier: For characters with a share of advantages and weaknesses that make them Top 8-viable but would require outstanding skill and effort to score especially high in National settings. See; Luigi and Yoshi in Melee.
Borderline Tier (Lower Mid): For characters that have noteworthy strengths, but have significant weaknesses or are generally outclassed by the higher levels of metagame. Unlikely to top nationally as a result of being mained, and often will be most relevant as counterpicks or secondaries. See; Young Link and Ganondorf in Melee.
Low Tier: For characters that have more weaknesses than strengths who suffer when trying to keep up with higher level characters. Even high-class players may struggle to use low-tiers effectively, but they may have one or two workable matchups.
Bottom Tier: For characters that are mostly non-viable and have substantial weaknesses that severely outweigh their strengths.
Negligible Tier: For characters that are essentially completely nonviable at top-level play even with extremely good players backing them due to a variety of weaknesses that're often related to the game engine itself, atrocious frame data, a near-unworkable moveset, or bugs. See; Kirby and Bowser in Melee, and The Triforce Trio and Jigglypuff in Brawl.
Mii Fighters:
I don't think they can be adequately estimated right now. Neither Gunner nor Swordfighter are realistically worse than several people in the lowest ranks, but they lack results or a lot of play, and the rules applied to them are inconsistent. The latter is probably the biggest issue and could directly affect the former two. Hence, for now and until we get a universal ruleset regarding them, I won't be categorizing them.
And some reasons
Ryu and Meta Knight's high placement: Very good results in the past few months, especially Ryu. I think Ryu could sit up with Sheik and ZSS in due time if he continues to do well in tournaments.
The order of high tier: This was extremely difficult to make. Pikachu doesn't even have results outside of ESAM's use, and much the same can be said of Mario. Yet, this applies to Rosalina. However, Dabuz very consistently places within the Top 4, while ESAM and Ally sometimes struggle to reach that in more heated competition.
All of mid-tier: A very rough and undefined order, as all of these characters have similar results. They do well on occasion, but never seem to break out, hence I place them in the same mid-level category. In comparison, Upper-tier characters tend to do more consistently well at a top level.
Lucas in borderline: Newly discovered Nair SDI weakness led me to bump him from Mid to Borderline. If one of his key combo tools is really that exploitable, he may go lower in the future.
The rest of Borderline: All characters who are agreed to have some major weaknesses yet give out respectable results/gameplay in tournaments. This is definitely a temp position; I think these characters with either migrate upwards or downwards as time goes on because either their sudden stir of relevance was due to unfamiliarity with the matchup or they really do have hidden strengths that need to be tested by a higher caliber player.
Specific tier for Jiggs and Zelda: It seems as those these two in particular would be hard to make workable/viable without a high number of buffs. In Jigglypuff's case, I feel like the game engine as a whole has (since Brawl) not catered to her gimp-heavy gameplay. I feel like she lacks even a basic core as a result, so I ranked her below Zelda. You could argue Zelda doesn't either, but I feel like Zelda has a "plan" as a character that just needs buffs. Nonetheless, both are garbage atm.