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Hi. I think we could write/use the phrase "some of the best coffee," but we couldn't write/use the phrase "one of the best coffee." Why is that?

I think what comes after the phrase "one of the best" should be a noun that is countable like "one of the best coffee brands."
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Both are perfectly natural.
The uncountable "some of the best coffee" is comparing X to all other coffee.
The countable "one of the best coffees" is comparing X to all other brands or strains. The word "brand" etc. is optional, but less often used these days.
Comments  
You have the answer already, you just haven't acted on it. Because "one of the best" is countable, you need the plural, coffees. Maybe one of the best coffees in the world would be more natural.
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Hi. Thank you. Could we write/say "some of the best coffee"?
 Avangi's reply was promoted to an answer.