General:
* Accord
* Agreement
* Concord
* Unity
* Compatibility
* Congruity
* Balance
* Coherence
* Consistency
* Smoothness
* Order
In music:
* Melody
* Euphony
* Consonance
In relationships:
* Amity
* Fellowship
* Camaraderie
* Peace
* Understanding
Other:
* Symmetry
* Balance
* Proportion
* Order
* Equilibrium
The best synonym to use will depend on the specific situation and the nuance you are trying to convey.