I made the best brownies today!
This is an example of an unreliable narrator because the narrator does not know if this is true. She has never eaten every single brownie in the whole entire world, so how would she know if the brownies she made were the best. The rest of the sentences are all facts. The last sentence, even though it shows her affinity for brownies, does not make her an unreliable character because she is just stating her preference.