That anime you will keep finding yourself coming back too, and watching over and over again. It may not even be your favorite or one you particularly like all that much but regardless you find yourself watching it when you dont feel like watching anything else and just need something comfortable and familiar to put on.
For me that anime is Masamune-kun no Revenge (Masamune-Kun's Revenge) It dosnt rate up there among my favourite shows and its not even a contender, theres nothing particularly special or stand out about it yet for whatever reason its the one show I have re-watched more than any other.