General antonyms:
* Agreeable
* Conciliatory
* Cooperative
* Harmonious
* Peaceful
* Submissive (this one has a negative connotation)
Antonyms focused on communication style:
* Conciliatory
* Diplomatic
* Non-confrontational
* Peaceful
* Reasonable
* Understanding
Antonyms focused on attitude:
* Accommodating
* Amenable
* Compliant
* Docile (again, has a negative connotation)
* Passive
* Submissive
The best antonym will depend on the specific context in which "argumentative" is being used.