Hardest:
Easiest:
Explanations for Hardest Characters:
I consider Olimar difficult because his pikman management and RNG that is involved with that character in addition to learning how different characters are combed based on what pikman you have, what throws combo into what based on pikman, and overall matchups stands to make him one of the hardest characters in the game.
Lucario is difficult in the sense that he is unusual for a smash game and feels like he was taken straight from Street Fighter or MvC. As such he can be difficult in the sense that it requires learning his specific cancels and combos in more of a traditional fighting game sense rather than a Smash sense which can seem difficult to those not familiar with traditional fighters in general.
Lucas and his ATs are one of the most difficult to master in all of PM and have some of the biggest payoffs in PM much like Fox did in Melee. He is very difficult to pick up and play on a competitive level immediately but he can be dangerous in the hands of someone very skilled with him, as it should be based on the design goals of PM.
Snake is also a different character because much like Lucario, is a character that does not play like a Smash character at all. General concepts like stage control apply to Snake but in a very different way. Mine management, B reversing Grenades/Tranqs/Sticks, C4 Placement, and Grenade management are all vital to the character wining the match let alone being competitive. Snake plays a very cerebral game of smash that is not so much taxing on the fingers as say Lucas, but more taxing on the mind.
Squirte has his own specific brand of technology called shell shifting which is merely scratching the surface of the amount of technical mastery required to play that character. The only other character that has specific technology like that (Fox) is also included on this list.
Pikachu has a lot of difficult movement tricks and quirks that are not as difficult as say Lucas but can be a stumbling block for many people who are new to the series or competitive scene.
Falcon, Fox, and ICs are hard in their own respects but for the sake of brevity I will simply refer you to their Melee character discussions (Although IC's probably don't apply as much to that in PM) to give you an idea of how difficult those characters can be.
Explanation for easiest characters:
The easiest characters have a number of things in common:
-Flow chart combos
-Low technical floor
-Can pick up an play easily and be competitive right of the bat
-Elevating your play requires a greater understanding of fundamentals rather than technical mastery
Because of this I believe that Shiek, Marth, and Mario are one of the easiest characters to pick up and play right away and be good. At higher levels however, technical mastery will be required to win consistently with these characters just because they are easy does not guarantee that they will win.