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DominikABBA is the best and well knownI think that Abba has to be considered as a 70's
ABBA is the best and most well known / best known.Mamma mia, do I know them!
Mister MicawberABBA is the best and most well known / best known.Why can't I say well-known ?
ABBA is the best and well knownI suppose you could, but your original sentence reads to me as if the article marks two superlatives but is followed by a superlative (best) and a base adverb (well); in any case, it is ambiguous.