an adjective (the Superlative).
The sweetest part of a watermelon (when it's not cut or sliced) is the middle of the fruit.
The skin
The Sweetest Part - 2013 was released on: USA: 19 October 2013 (San Jose International Short Film Festival)
The cast of The Sweetest Part - 2013 includes: Dana Byrne as Xiu Leann Lei as Lian Alice Wen as Ting Wang Glenn Zhang as Dad
That is the sweetest thing! He is the sweetest boyfriend I've ever had. Candy is the sweetest of those foods.
part of speech
The part of speech for this particular word is a noun.
That is the SWEETEST thing. You are the sweetest. etc.
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "explicit" is an adjective.