My recommendation: "Mango A-Go-Go" and the "Sweet Street Waffle"
The absolute best thing you can order at Jamba Juice is ironically not a juice, or a smoothie. The Sweet Street Waffles are a godsend. I suggest getting them toasted and possibly get two. They're so good, it's hard to just have one.
The smoothies there are very good. They try to stay away from adding sugars or syrups, although most of their juices are still made from concentrates.
On a side note, if you are a super health nut, Jamba Juice's wheatgrass seems to taste sweeter than other smoothie places that sell it. I've had "bitter" tasting grass from other smoothie chains which make taking wheatgrass (which has a hard-to-bear taste already) even more of a chore, whereas Jamba's wheatgrass is a tad bit easier to "swallow".