how do you define "hardest to play"?
if something is "hard to do", that means that very few people can do it.
so the "hardest character to play" would be the character that the fewest people play.
BUT, the tier list needs to be factored in. more people will play characters that are good in tournament.
so, the hardest character to play is the character with the least people that play them, relative to their spot on the tier list.
for example, lots of people play falcon even though he's 7th best on the tier list. this indicates that falcon is an easy to understand character compared to a difficult character like jigglypuff.
much more people play falcon than jigglypuff, even though jigglypuff is a better character for tournament play. this is because jigglypuff is too difficult for most people to play.
other common characters in tournaments are fox, falco, and sheik. these are easy to play.
uncommon characters (which are still good) include marth, peach, and ice climbers. these characters are difficult to play.
say what you want about tech skill or knowledge, but the popularity of different characters speaks for itself. i could teach a monkey to waveshine.
there are other factors such as precise spacing that make puff and marth (for instance) more difficult than they seem at first glance.