Sekali and Banget have the same meaning. The difference is that Sekali is somewhat formal and Banget is informal. So : Bagus sekali = Very good, for formal situation (when speaking to older people, parents, to your boss) Bagus Banget = Very good, for informal situation (when speaking to your friends or family)

Just added explanation If you mean : "sekali" is superlative words, yes its true eg : Dia cantik sekali. ( She's very beautiful) but "sekali" not only "superlative words" Same word in English "sekali" = Once eg: Sekali lagi, maafkan aku. (once again, forgive me)

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