Best object placing tool today: ObjectPlacementTBH
Object placement tools list (in no specific order):
Eden is built in arma3 feature no other software are needed. Map Builder, ObjectPlacementTBH and X-Cam are addons. Terrain Builder is something you are already aware of (or at least should be if you're reading this heh).
We haven't been able to test M3Editor because its installation is tied to installing “a3launcher”.
If you have personal experience using eden, m3editor or surface painter, please contact us.