Since I don't drink tea or coffee, I buy cordials by the bucket load. My absolute favourite is Lime and Lemongrass made by Belvoir - but it's quite hard to come by. Not sure about sugar content, but it doesn't taste too sweet (I have a sweet tooth however). Belvoir do a whole range of interesting adult cordials - expensive, but since I don't do tea or coffee, worth it!
Second favourite is cranberry and hibisicus by Thorncraft - this can be found in most delis - again quite sour and not too sweet.
If I'm struggling to get hold of those, then Duchy Original strawberry and mint cordial is available in Sainsburys - but it is packed with sugar. Once opened, keep in fridge because it moulds up quickly otherwise
