Per drink, the best thing to drink on a diet is vodka with a diet mixer. Per ml it may contain more sugar or calories, but per drink you drink a smaller amount than other drinks therefore consuming fewer calories
I've also heard champagne... I recently went to London on a date and was told by my date that this was true... but at �450 a bottle, I wasn't too bothered about how many calories there were, we ordered 3 bottles and downed the lot... x
If you like wine, treat yourself to a WeightWatchers bottle of wine, they are really nice and available from most leading stores (TESCO, ASDA etc). they contain less calories/sugar than most other.