The theme song might claim "the fun will never end," but the Adventure Time TV series eventually did come to a conclusion. Throughout the series' run, however, protagonists Finn and Jake encountered a litany of idiosyncratic characters, many of whom were spectacularly strong. While Finn is the reincarnation of a powerful Catalyst Comet, he's merely human in his current life. As such, his victories against powerful foes come from a mix of teamwork, luck, and weapons (cursed or otherwise). While Finn may be the star, he's not actually one of the strongest Adventure Time characters. On the contrary, there are several more powerful than him.
Source:https://www.cbr.com/adventure-time-characters-ranked/