Superlative form of silly: most silly. (adjective)
Wiktionary.org : Text is available under the Creative Commons Attribution-ShareAlike License
Use "silliest" in a sentence
"Days before the repeal effort was announced, he came up with yet another way to apply the word silliest to the ban, saying: I think that was one of the silliest, silliest decisions anyone made, to outlaw foie gras."
"It is an artistic treasure; but it glorifies a murder which Goethe described as the silliest crime ever committed."
"John Chapman, Bishop of Ottawa for coining the silliest phrase of the year: experiential discernment: trying to decide whether doing something is right by – doing it."