For me it was a hard choice between the latter two. When exhibited right, a family group of giant otters are extremely entertaining. The family that LA had the last couple years (I think most of the young have been shipped out to other zoos now) would all ride the water slide one after another, chatter about it to each other, twist and twirl for the visitors, then run back up to start again.
That being said, I think there's fewer things that amaze zoo visitors more than a singing siamang duet. At every zoo I've been to that has them, they can be heard throughout the premises; when they start singing, everybody stops what they're doing and wonders what it is they're hearing. Their exhibit can also draw big crowds at the time.