I've always found that the top tiers are always clear cut, some characters (Fox) are better than others (Falcon), however the mid and low tiers are where most of the debate happens. Due to a lack of familiarity with each character in the lower tiers, it can be hard to accurately rank these in a way everyone can agree upon.
With that being said, I will present my own opinion with an explanation of each placing so we may understand why these characters have these placings (unlike the previous posts here).
https://smashboards.com/tiers/#1,10...,3,25.F#19,15,21.G#17,26,1,10.1,1,1,1,1,1,1,1
Fox, Falco, and Marth reside in the S tier. I wonder why.
In A tier, I placed Puff above Sheik, primarily because of Sheik's many fewer ways to deal with camping and crouch canceling by all characters. Nevertheless, Puff, Sheik and Peach are still very powerful in tournament play.
Next comes Falcon and ICs. Gee, I wonder why.
In the C tier we have Samus, Pika, and Luigi. While all of these charactars have a fair amount of top level representation, they don't quite make the top cut because of a few noticeable flaws (Pikachu's great ending lag and Luigi's slipperiness to name a few).
D tier is where I'd call MID tier. This includes Ganondorf, Mario, Yoshi, and Doc (in that order). Ganondorf is a true mid tier, slow but powerful however still has movement options and ways to deal with most situations that any character could throw at him. Mario was places right after this due to his large representation in the higher levels of play, along with winning matchups with most characters below him on this list. Yoshi comes next due to his movement, representation, and is held back by his lack of a recovery. Next comes Doc. While most people would place him above the mentioned, and while his potential is likely that high, there has been a massive lack of representation of Doc since Shroomed dropped him in 2015. While some doc mains such as Palika and Kevikoran do place well, they don't attend enough national events to make a case for Doc's threat in today's meta.
E tier is likely the most controversial, though all of these are very similar in viability (which is low-mid tier). G&W was placed at the top of this tier largely due to his frame-positive actions on shield and high-level representation. Next is Link, which has very much representation in the mid levels of play but few links above that. His bomb physics and his overall power placed him above Yink. However buffered between these two is DK, who has high level representation. He has spacing tools and ways to beat CC, which is very important in today's meta. However his weight and overall slowness keeps him from placing higher. Now we have Yink, who's known for his MU with Puff. While it's debatable if it's good or not, we can see that Yink has gotten top level representation in recent time (see Axe vs HBox and Mang0). While Yink has speed and many good projectiles, his unfavorable move physics keep him from outplacing the other characters in this tier (see his multi-hit up-b and bomb knockback directions).
F tier is low tier, where character knowledge for typical player dwindles. I placed Pichu at the top of this tier, followed by MewTwo and Roy. Why Pichu so high? Pichu is the fastest of all of these characters (with extra techs such as PerfectAgility), however lacks a reliable kill option. Not only this he also is the lightest character in the game, which makes him die early. But his strengths outweigh his flaws in this ranking. Next is MewTwo, who is known for his battlefield side-b gimps. He like most low tiers lacks a way to deal with cc, though his combo game is strong vs floaties such as Peach, mostly due to his DJC and nair. MewTwo's wonky phisics and uniqueness makes him scary to fight without mu experience, and perhaps this is why we can see moderate success at a higher level of play from certain mewtwo mains. After M2 is Roy, who has lack of ways to deal with cc, unfavorable recoverer, and badly places sweetspots. However, with excellent spacing, charactar knowledge, and gimps, we see success with Roy when wielded by top players (see Leffen). But don't let this fool you Roy is trash
G tier is just so bad charactars. None of them can deal with CC in any way, don't have any favorable MUs with charactars above them, and rely on one or two moves. Ness has bair, Zelda has bair, Boozer has up-b oos, and Kirby has upair at best. They all suffer from lacking recoveries, lack of punish game, and slow movement. Ness is the best of these, due to his own DJC, but even then he can't kill for ****.
Anyway that was my explanations. They'll probably hard to understand and leave you with questions, but that's no different from the questions I had when reading the other's lol.