Hello,
Unfortunately, Cesium doesn’t currently have support for z-ordering. We have an issue written up on our GitHub here: https://github.com/AnalyticalGraphicsInc/cesium/issues/4108
I added a link to your forum post so we can track how many users are requesting this feature.
For now, you can either use an alpha that’s very close to 1 to make the polygons translucent, or you can try to add a small height to the polygons.
Best,
Hannah