These types of threads are made all too often and the OP always fails to specify what they mean by "difficult".
IMO, "difficult" can mean several things. Are you referring to technical difficulty or just raw difficulty? If you are referring to the latter, a lower tiered character will always be harder to play than a higher one just by virtue of the fact that character has less options/worse MUs. Yeah, sure, Fox and Falco have a lot of APM, but playing Falcon is wayyyy harder at top level because he has a terrible recovery, less neutral game options, some terrible MUs, etc.
If you are talking about technical difficulty, then again, this could mean several different things. Some characters have high tech floors (necessary tech skill to compete) while other characters have a higher tech ceiling (potential tech skill). For instance, Fox/Falco/Yoshi probably have the highest tech floor; your movement, shield pressure, and character specific techs require a lot of APM and consistency at top level. However, if we are talking about tech ceiling, the Ice Climbers totally eclipse every character in the game since desyncing has inconceivable implications.
And yet again, you could be referring to combination of technical/raw difficulty; an overall difficulty if you will. This would probably consist of your character's tech floor, options, and MUs. This approach would probably be the most informative, which, if that is the case, the list would probably look like this:
Overall difficulty when looking at "viable" characters:
1. Ice Climbers (These characters are very overlooked in terms of difficulty. Yeah, wobbling might be a rhythm, but these characters are not intuitive to play in relation to the rest of the cast. Desync tech has a lot of depth and while shine stuff might have more overall APM, desyncs are far more situational and require more awareness from the player. Furthermore, ICs suffer from a lot of bad MUs. Peach, Falcon, Samus, Ganon, Marth, Fox, can all counter them to some degree so going up against knowledgeable players are top level will be extremely difficult)
*2. Falcon (High APM, tough MUs, polarizing flaws, etc. You have to be on your A game and use reads to find success at top level)
3. Fox (Has a very high APM and high tech ceiling which makes him difficult to be consistent with. With that said, he has a lot of great MUs and I don't think he would be harder than the two characters listed above him.
4. Falco (Self explanatory, he has less APM than Fox and a simpler neutral game)
5. Pikachu (Doesn't have a lot of character specific techs but pretty high APM overall. Some bad MUs)
6. Samus (ugly MUs despite very low APM)
**7. Marth (Pretty overlooked in regards to difficulty, IMO. He doesn't have a lot of APM but he is very reliant on spacing and situational awareness. Pretty hard at top level)
8. Peach (Pretty low APM but she does have some hard MUs)
9. Sheik (Good MUs, lower APM, great options, still hard to play at top level like any character but relatively easier than the aforementioned ones)
*If you consider Yoshi viable, put him here and bump everyone down.
**If you consider Doc/Mario viable, put him here and bump everyone down.