The Cosmopolitan was originally made with citrus vodka, but it's great with unflavoured vodka or a combination of the two vodkas - my preference. For this...
I used straight (unsweetened) cranberry juice. Enjoyed the crisp tartness. Far cry from the Cosmo's of my youth which were uber syrupy sweet. While using a sweetened cranberry juice has it's place, I prefer the briskness and pucker of straight up cranberry.