Easiest to play is probably Diddy in terms of easiest to play and easiest to play at a high level. Easy followups from grabs, easy initiation with banana, His moves are all pretty straighforward with natural-feeling hitboxes, range, and speed. He's also got a projectile and his recovery is amazing.
Hardest to play might be Wii Fit Trainer since her play style isn't straighforward, her hitboxes are awkward and have different properties whether you hit the opponent with the front or back of a move (ftilt for instance). Not to mention, she has to charge a projectile, charge a buff for the projectile, then use the projectile in order to get early-ish kills since her kill potential on a lot of her moves isn't that great and her smashes are slow, have small hitboxes, and have a ton of endlag on them making them unsafe.
Hardest to play at a high level would probably be Robin. You need to know when to use the bronze sword and when to use the Levin sword since the bronze sword has less KB, comboing with it is easier but those combos are only effective until certain percents and all those percents are different for every character. Then you need to keep track of how many uses your spells have so you can catch the book when it's thrown away because utilizing those books is absolutely necessary for high level Robin play. Plus, you need to know the timing on a book's recharge so you don't activate an empty animation. Robin's also got some weird hitboxes on his aerials such as fair and bair which both start from below him and rise up slightly you have to be above the opponent in order to hit with the first hitbox frame. Plus, all of Robin's aerials, if not smashed, will use the last sword Robin used which means keeping track of the last attack you used in order to effectively use the type of aerial you want, especially if that aerial is a bronze sword attack that isn't nair because you have to use a bronze sword attack before doing that otherwise, it'll be a Levin sword. Plus, using nair to reset aerials to use the bronze sword is a thing. Robin is, by far, the most technical character meaning that using him at a high level requires use of all the technical bits about him.