My two cents: Sheik is the best character. She is also extremely technical compared to the average character and her lack of KO power makes her very difficult to play at a consistently high level. But she has a fantastic recovery and the best frame data combined with needles giving her the best neutral game. But that's if you make very few mistakes. She also has the best matchup spread in current meta as she doesn't have any straight up "losing match-ups" but the top competitive Sheik players are picking up secondaries which shows how difficult it is to win with her.
Mario is probably "the best all-around character." Sheik struggles to KO, Cloud has a terrible recovery, etc, but Mario has great frame data and no really bad weaknesses and he's good at everything. ZeRo has been pulling out Lucina a lot lately and he's a Diddy main. Ally seems to be able to use Mario as a solo-main. At least with more success than Bayo, Diddy, Cloud, Rosa and Sheik mains.
There's an argument for a lot of characters as "the best" but it depends on what are the parameters of 'the best' title. For glory win percentage? Tournament wins? Match-up spread? Frame data? Eventscrubs? Trying to consider everything I'd say Sheik, Diddy, Bayo, Rosa, Mario and Cloud have some argument.