The Cosmopolitan was originally made with citrus vodka, but it's great with unflavoured vodka or a combination of the two vodkas - my preference.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...
The best way is shaking the Cosmopolitan with some lemon zests, that way you have the lemon flavor aswell but it is more refreshing and taste more natural than citrus/lemon vodka.
A tasty cocktail. After reading the comments I added a barspoon of sugar syrup to approximate Simon’s cranberry juice.
The Cointreau really ‘sings’ in this drink.
I used Ocean Spray Cranberry Classic. 5g of sugar per 100ml, and 27% cranberry juice. The Cosmopolitan using this was delicious, perhaps a little bit on the tart side. I found it available in small 200ml cartons in a package of 3 which makes it really easy to store and use without having to buy a large bottle.
I use "Eager Naturally Sweetened Cranberry Juice Drink" with 14.1 gram sugars per 100ml. Importantly, this also contains 26% cranberry juice and 1.5% cranberry puree. Some other brands have a very low percentage of cranberry.