I absolutely love American Mountain Dew. Every time I go to the USA, it's virtually all I drink. It's terrible for your health, but it's so much better than the stuff we have over here ; it's like night and day.
Another US soft drink which I would love to see widespread over here is root beer. As I live in London I have (fairly) easy access to it, but I would love to see it sold elsewhere. In terms of big brands, I think the nicest is probably Mug.
Special mention goes to Mezzo Mix. The first time I had it I just though 'meh, orange coke', but now I have a bit more of a craving for it.