Basically everything is must see! Apart from the Orangs, definitely don't miss the Proboscis Monkeys (two exhibits), Douc Langurs (an island) and the Critter's Longhouse (small mammals). All these are clearly shown on the map, so you shouldn't be able to miss them. In fact if you go down all the paths shown on the map you should see everything, but there are some free-ranging primates that might be harder to spot.
Are you going to any of the other zoos in Singapore? River Safari and Night Safari are adjacent to the Zoo, doing the latter after the Zoo is possible, but its a long, hot day. Better than missing it though. Jurong Bird Park is also amazing, and the new SEA Aquarium on Sentosa Island is supposed to be very impressive.