I second the vote for the Botanic Gardens and Sungei Buloh. Add to that your decision for Fort Canning. These 3 together will give you a very nice slice of Singapore.

Botanic Gardens - As much about history as anything else. The canonball tree, the rubber saplings ...
Sungei Buloh - Mangrove and wetlands, fast disappearing all over South East Asia
Fort Canning - Early colonial history, cemetery

Bukit Timah and Macritchie maybe fairly similar experiences. I think they technically touch each other and you can hike from one to the other if you have a good map. Labrador I would pass but that's just me.

Another idea if you get a chance is perhaps Pulau Ubin (Ubin island) as that is quite a "park" too.