Always buy juice made NOT from concentrate. Copella is a really good make, and is made only from fruit- no added sugar or anything nasty. Fruit juice is still high in sugar and can whack on calories if you drink it by the bottle, so it's best to just have a small glass a day.
p.p. s I would not recommend diet drinks at all- they contain aspartime and other nasties that are probaby carcinogens if you have enough of them. Avoid!