Idioms are expressions that have a figurative meaning that is different from the literal meaning of the words. For example:
* "Kick the bucket" means "to die".
* "Spill the beans" means "to reveal a secret".
"My favorite" simply means the thing or person that someone likes the most.
Perhaps you are thinking of a specific idiom related to a specific topic? If you can provide more context, I might be able to help you find the idiom you are looking for.