General Synonyms:
* Tactful: Showing skill in dealing with people in a sensitive way.
* Discreet: Prudent in one's speech and actions, especially with regard to confidential matters.
* Skilful: Having or showing expertise in a particular field.
* Adroit: Clever or skillful in using words or handling a situation.
* Negotiating: Engaging in discussion with the aim of reaching an agreement.
More Formal Synonyms:
* Negotiatorial: Relating to negotiation.
* Ambasadorial: Relating to an ambassador or embassy.
* Consular: Relating to a consul or consulate.
Informal Synonyms:
* Smooth: Operating without difficulty or disruption.
* Politic: Prudent and tactful in one's dealings with others.
* Slick: Skilled and clever in a way that is often thought to be dishonest.
Negative Connotations:
* Devious: Showing a skillful use of underhanded tactics to achieve an end.
* Machiavellian: Cunning, scheming, and unscrupulous, especially in politics.
The best synonym for "diplomatic" will depend on the specific context.