For as long as amiibo training has existed, there has always been both top and bottom-ranked characters. In Super Smash Bros. 4, heavyweight fighters like Bowser and Ganondorf ruled the early and late metagames; in Super Smash Bros. Ultimate, the top-tier fighters are more diverse and can’t be categorized into a single description.
Super Smash Bros. 4
In Super Smash Bros. 4, equipment was both allowed and encouraged and was included in the standard ruleset. Little Mac and Cloud were once banned, but were later permitted under a strict set of equipment restrictions. This tier list accounts for both of these restrictions; they are still top tier, but are not the best characters in the game.
Super Smash Bros. Ultimate
Super Smash Bros. Ultimate does not yet have an official tier list, but a general idea of the best and worst characters has been formed. The best amiibo in the game is Bowser; he is so good, in fact, that many trainers do not enter him because a properly-trained Bowser is almost guaranteed to win tournaments. Other top-tier characters include Olimar, Piranha Plant, Link, Mii Gunner, Lucina, and King K. Rool; however, with the exception of Mii Gunner, none of these amiibo are particularly easy to train and still require much work.
Among the worst amiibo in the game include Bayonetta, Sheik, Inkling, and Zero Suit Samus; these results were calculated via statistics of tournaments that have been held up until this point. All of these characters have rather poor kill power or AI that holds them back from tournament viability. When an official Super Smash Bros. Ultimate amiibo tier list becomes available, it will be updated here.