Super Smash Brothers 3DS

Tiers are pretty subjective and what they're based off of is also subjective to community feelings about a character it's good to have an idea of the potential of a character in terms of expected results amongst equally skilled players. Before I get comments of bias I'm a Marth main who has primarily been using Lucina since the release of SBB4 my secondary is firmly Peach followed by Mr. Game Watch. To explain the low ranking of formerly high tier characters like Marth you have to look towards the deterioration of the importance of an aerial game which is precisely what Marth excelled at, since it's rendered fairly trivial in SBB4 he was likely booted to being trivial. I included the reasoning for some of the stand-out cases, but didn't feel the need to explain all of them.
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S
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Robin - The Damage potential, Knock back, and versatility firmly makes Robin one of the best characters out there, the only real flaw is a steep learning curve.
Zero Suit Samus - Speed, Range, and Plenty of combo-ability puts ZSS right on Robin's tail for best character.
Rosalina - Rosalina is like the Ice Climber except even more easily de-sync'd and Luma respawns unlike Nana. Rosaline herself is quite formidable, but with Luma she's just that much better.
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A
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Shulk
Sheik
Greninja
Yoshi
Duck Hunt
Sonic
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B
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Fox - Fox formerly relied on strong airs for easy KOs, but with the decline of aerial game he lost much of his quick KO potential, he can still deal damage, but his knockback leaves him wanting.
Mario
Dr. Mario
Wario
Peach
Bowser - Bowser got a massive speed boost and still has very imposing weight and knockback, Bowser finally feels like the Koopa King he should have been.
Wii Fit Trainer - Unorthodox fighting style has people having difficulty predicting playing the enemy rather than being played herself, she'll go downhill fast once people learn her attack patterns since her unorthodox movements also leave several blindspots and limited hitboxes to attack with.
Meta Knight - Meta Knight lost his top-priority and had several weakened attacks and a speed reduction, the pitiable reach stayed however dropping his standing way down. He's still Meta Knight so expect quick damage, but anticipate a hard follow through to score that KO.
Ike - Weight and knockback have become keys to success in SBB4, Ike has that, but his speed hinders him still.
Little Mac - Little Mac is predictable and punishable despite his outward appearance, once you get used to his attack patterns you'll find him to be an average character that can be easily countered. His air game isn't his biggest flaw, it's his predictability.
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C
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Diddy Kong
Lucario
Pikachu
Samus
Dark Pit
Pit
Link
Luigi
Captain Falcon
Rob
Bowser Jr.
King Dedede
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D
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Charizard
Toon Link
Mr. Game & Watch
Kirby
Donkey Kong
Zelda
Ness
Jigglypuff
Olimar
Marth - Aerial games are trivialized and without it Marth simply doesn't hold up. His damage is well below average on the ground and his main kill shots were formerly aerial spikes or follow off forward airs, both have been decimated in terms of effectiveness. His ground kill shots are fairly predictable and lack the bluster of other characters couple that with the need to get hit with a disjointed hitbox and you have a very sub-par character despite seeming to play like his former self.
Falco - Falco is laughably bad, he was hit across the board and left with practically nothing in his favor in terms of his former self. You still have some tricks which keeps him from being the bottom of the barrel.
Villager - The Villager is complicated and his stronger attacks can be reflected by a myriad of characters, he has that potential to be good, but it's not realized through his glaring flaws.
Pac Man - Like Sonic Pac-Man has trouble damaging and killing an enemy unlike Sonic you don't have that outstanding hit and run capable play style.
Lucina - Lucina is just plain worse than Marth, there's no way to sugar coat it. Where Marth can have the extra power for skilled players Lucina trades off for sub-par power that's consistent throughout her reach and it's not even close to a fair trade-off when Marth can utilize the disjointed hitbox in his favor in terms of using the weaker parts to setup combos and then a tip to kill. Like Marth her aerials are her selling point, but they don't matter terribly, she is slightly faster than the Hero King, but is left more vulnerable after a missed strike. If they left Roy in the game he'd have been a much more interesting option and would have easily surpassed Marth in this iteration.
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F
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Palutena - Palutena suffers from a poor hitbox and low damage, she's fairly slow, and she's predictable. Her hitboxes don't necessarily match with the key inputs making her hard for beginners to use in addition to all her flaws, there's no wonder she just supports Pit in Kid Icarus.
Ganondorf - Ganondorf received various buffs throughout the ages, but his original form would have been more at home in this diluted installment of the series, he has power still, but unrealized potential all around.
Mega Man - Mega Man is the dreg of SBB4 which is saying a lot. He can't utilize his mobile standard attack effectively which would have gave him a large advantage and he has numerous flaws such as reflect-able Smashes delayed specials and all around slow predictable attacks.