These boots are my favorite.
These boots are my favorites.
Should I use "favorite" or "favorites"?
A twelve-year-old girl is expressing herself. The idiom includes an unspoken word, which could be "thing".Also, in one article they wrote
”Collars are my favorite.”
Why is this ok and my example wrong?
Using the expression, you could say Boots are my favorite! I'd consider the exclamation point to be a necessary part of the expression. I don't recommend that learners use this formula. It's casual usage which could easily look out of place.