you may say either. In my American dialect, the most common and polite way would be "Could you do me a favor?" This is slightly different from "Can you do me a favor". using "can" is much more common in practice, but the polite form is "could", since it gives the person the option, the choice. might want to throw in a please there as well. (could you please do me a favor? or could you do me a favor please?-the first is more of a pleading tone than the second)