there are too many to choose from - coffee, mint chocolate chip, and classic chocolate are probably my favorite. and cream and sugar is up there somewhere too.
i love me a basic chocolate shake, personally or chocolate peanut butter. or any variant of basic chocolate really, the food chain sonic used to have what they called a chocolate cream pie shake which was a chocolate shake with I believe graham cracker crumbs and coconut cream (and extra whipped cream on the top) and even though i haven’t gotten one in years i still think about them constantly