"The beans that you prepare are the best."
Translation:Les haricots que tu prépares sont les meilleurs.
Mieux is usually an adverb (but see the link below as it isn’t always). Meilleurs can be an adjective or a noun. Here I think it is a noun, referring back to the word haricots. Le mieux and les meilleurs both translate as ‘the best’ In English, so you need to know your adverbs from your adjectives and nouns. https://www.collinsdictionary.com/dictionary/english-french/best https://www.collinsdictionary.com/dictionary/french-english/mieux