In gaming, a tier list is a list that ranks all characters in a game based on the strength of their fighting abilities, as well as their potential to win matches under tournament conditions, assuming players are of equal skill. Tier lists are most commonly made for fighting games that are played at a high competitive level, though games with large character pools, such as the Pokémon series, can also have their own tier lists.
A large number of different tier lists exist for the Super Smash Bros. series, but the most widely accepted tier lists have generally been produced by the Smash Back Room on Smashboards. Exceptions, however, do exist, with the first widely accepted tier list for Smash 64 being created by users on GameFAQs, and the most recent Smash 64 and Melee tier lists being created by input from fan votes.
How tier lists are calculated
A character's standing on the tier list is based on a variety of factors:
The current collective metagame
The metagame's current state involves what tactics, characters, stages, and other factors make up the most common "decisions" made in tournament play; in other words, how the game is generally being played (fast, slow, powerful, combos, etc). In the tier lists, these decisions help to rank characters, based on how useful they may be in tournament matches shaped around these decisions. Metagames based on either the players' collective actions or a tournament organizer's rulesets can result in characters being ranked higher or lower on the tier list. Powerful but slow characters who are easily KOed are typically lower tiered. Faster characters with quick attacks are normally higher tier.
An example of the collective playerbase's metagame affecting the tier lists can be seen in Brawl's tier list and its evolution over the years; on the first tier list, R.O.B. was considered among Brawl's best fighters, ranking sixth, owing to his long, safe recovery, powerful projectile game, and a number of excellent edgeguarding options. As Brawl's metagame began to advance, however, Meta Knight's incredible potential led to him becoming the most used character in the game's tournament scene by a large margin, with a notable part of each character's metagames involving how well they could handle the near-ubiquitous character. R.O.B. was later noted for having a very unfavourable matchup against Meta Knight, due to being too slow to allow his projectile game to take hold, as well as Meta Knight having an extremely long, safe recovery that is notoriously difficult to intercept. R.O.B. now ranks as 18th on the tier list, with his lowest position being 20th out of a pool of 38 to 39 characters. The increasing prominence of Meta Knight in Brawl tournaments also affected numerous other characters beyond R.O.B., such as Pit and Mr. Game & Watch, both of whom also struggle in the Meta Knight matchup and fell in the tier lists owing to this poor matchup.
For rulesets determining a character's viability, custom movesets in Smash 4 have demonstrated this in the game's tier list. With custom moves enabled, several characters gained powerful benefits, with Mii Brawler and Palutena especially benefiting due to their ordinarily lackluster movesets; some smashers even thought Mii Brawler was a top-tiered character with custom movesets enabled, as some custom moves allowed the Mii Brawler's approach and combo games to become much safer and stronger compared to its regular moveset. The ban on custom movesets starting after EVO 2015, however, has prevented the full potential of either character from being explored in Smash 4 tournaments. Both characters have thus far ranked poorly in Smash 4's tier lists, with Mii Brawler notably ranking 56th out of a pool of 58 characters in the game's third tier list.
The character's individual metagame
The metagame of individual characters is determined by how the players of a specific character use the character within the tournament setting, often in response to the general metagame. In general, characters with "deeper" metagames, or, more options in the game's overall metagame, rank higher than characters with "shallow" metagames, or, fewer options in the game's overall metagame.
In Melee's early history as a competitive fighter, numerous debates stemmed from the viabilities of Marth and his clone, Roy; opponents of the tier list generally used Roy as an example of how tiers were irrelevant, claiming that Roy's prowess in competitive Melee was equal to that of Marth. Ultimately, Marth has since been accepted as the far superior fighter, primarily due to the differing metagame statuses of both characters. Marth boasts considerable creativity in how he can combo, inflict damage, KO, and edgeguard opponents, owing to a safe approach from the distant sweet spot on Falchion, his down aerial spike, and a number of powerful finishers that can KO even if they are non-tippered; Roy, however, struggles to perform as well as Marth in these regards, as he has few viable finishers outside of his forward smash, the placement of the Sword of Seals's sweetspot prevents him from comboing or approaching as safely as Marth, and his off-stage game is considered among the worst in Melee due to his weak aerials and high falling speed. Melee's tier list has historically recognised these differences, with Marth frequently ranking as one of Melee's most potent fighters, while Roy has failed to rise out of the low tiers.
Outside of an individual character's inherent characteristics, responses to a character's metagame can lead to differing tier list placings. In Smash 64's first tier list, Ness was considered the third-best character in the game, due to his powerful double jump cancel combo potential; later analysis, however, found that despite the potentially high power of his DJC combos, his short range prevented him from easily taking advantage of this. Later tier lists thus ranked Ness considerably lower from his third-place finish, with the current Smash 64 tier list ranking him as tenth in a pool of twelve characters, owing to his low range, difficulty in winning the neutral game and poor recovery. Conversely, Jigglypuff in Melee initially started off as a mid-tiered character in the metagame, as it had some powerful attacks, but a slow, unwieldy ground game. Top professional player Mango, however, demonstrated that Jigglypuff's air game was among the most powerful in Melee, and he also demonstrated that Jigglypuff had surprisingly powerful combo and pressure games, beyond what most smashers had previously seen or expected. Mango and later Jigglypuff main Hungrybox began to take top placings at tournaments with Jigglypuff, with the duo collectively demonstrating how deep Jigglypuff's own metagame was, resulting in it jumping to the top tiers in Melee, including a third-place finish on the tenth tier list and a fifth place finish on the current tier list.
The character's matchup spread
A character's matchup spread analyses how well they can perform against other characters in the game. In general, higher-tiered characters have more matchups that are in their favour compared to lower-tiered characters, with matchups against higher-tiered characters having more weight behind them than matchups with lower-tiered characters. In Brawl, for instance, King Dedede has among the best matchup spread against characters that are lower than him on the current tier list (12th), and this matchup spread is actually among the best in the game in this regard; however, he is also countered by many characters that are higher-tiered than him, leading to his current placement on the tier list despite his favourable matchups. On the contrary, King Dedede has been considered potentially non-viable as a solo main character in the current Brawl metagame, due to his poor matchups against Meta Knight and the Ice Climbers, the top two characters on the tier list.
A matchup against a single character is generally not enough to significantly change a character's tier standing. In Brawl, for example, Toon Link, who ranks 13th, boasts a slightly favorable matchup against the Ice Climbers, who rank 2nd, owing to his ability to potentially avoid the duo's powerful chain grabs as a result of his various projectiles and disjointed hitboxes; despite this, Toon Link ranks 13th out of a pool of 38 characters on the tier list, due to having an unfavourable matchup against almost every other character that ranks higher than him.
Ultimate's competitive scene is still young, so the match up spread cannot be fully developed yet.
The character's tournament results
In general, higher tournament results for characters yields higher tier placings, as winning major tournaments implies that a character has more tools to compete, and thus, a deeper metagame. The top characters in all four tier lists have all maintained large playerbases and excellent results in tournaments, while characters directly below them also generally perform well or have large playerbases. Top-tiered characters are also most often used by the game's top players; the top players on both the SSBMRank, SSBBRank, and other power rankings are most often seen using top-tiered and high-tiered characters. Characters towards the bottom of the tier list, however, generally have smaller playerbases and poorer results, with some low-tiered characters lacking any notable representation in tournaments; Pichu, for instance, currently has no notable players that exclusively main it, and it has never finished in the top 96 of any notable Melee tournament.
Furthermore, an increase in results for a character can lead to a higher tier standing, as improvements to results can be attributed to metagame changes either in the entirety of the metagame or for an individual character. Characters such as Pikachu and Yoshi in Melee or Ike and Sonic in Brawl saw improvements in tier standing due to players discovering new techniques or applications for the character, yielding deeper individual metagames that could stand up in the general metagame; Ike players such as San, for instance, demonstrated that the character had a surprisingly powerful aerial game, alongside an extremely powerful and flexible jab that demonstrated Ike's neutral game was not as slow or unsafe as previously thought.
That said, tournament results do not necessarily equate to higher or lower tier standings, nor do larger playerbases yield higher tier standings. In Melee, Yoshi has attained some degree of success at large tournaments, having appeared in the top 32 of several major, international tournaments; the character's success, however, has primarily been buoyed by aMSa, and as such, Yoshi only ranks twelfth in a pool of 26 characters despite his incredible potential. Similarly, Salem's surprise victory at Zero Suit Samus in Brawl, Apex 2013, did not move her into the top tiers. While Salem used Zero Suit Samus's Power Suit Pieces to a level that was never seen before, his victory has been attributed to a lack of matchup experience from his opponents, rather than Zero Suit Samus being one of the best characters in the game. This was supported by the fact that Salem was unable to replicate his Apex 2013 level of dominance, failing to place in the top eight of any Brawl major before the release of Smash 4. As such, Zero Suit Samus remained a high-tier character following Salem's victory, since she was still considered less competitively viable than the characters above her.
Super Smash Bros. tier list
The following is the Super Smash Bros. tier list produced by the Smash 64 community. The numbers below the characters are their average ranking during the voting for the tier list. It is current as of May 12, 2015. 
Super Smash Bros. Melee tier list
The following is the twelfth Super Smash Bros. Melee tier list, which is also the first list to not be produced by the Smash Back Room, but rather by fan votes and feedback. It is current as of December 10th, 2015. 
Super Smash Bros. Brawl tier list
Super Smash Bros. 4 tier list
The following is the fourth Super Smash Bros. 4 tier list produced by the Smash Back Room. It is current as of December 11th, 2017, and reflects version 1.1.7 of the game. Pit and Dark Pit were voted on as the same character, while Mii Fighters were excluded entirely. Character customization was not taken into account due to their collective ban since EVO 2015. The tiers in this list were split into groups: Top (S,A); High (B,C); Middle (D, E); Low (F, G)
Super Smash Bros. Ultimate tier list