Formal/Neutral:
* Un bon communicateur (a good communicator) - This is a general term for someone skilled at talking.
* Un orateur habile (a skillful orator) - This emphasizes eloquence and public speaking skills.
* Un négociateur adroit (a skillful negotiator) - This highlights someone who is good at persuading others.
Informal/More Negative:
* Un charmeur (a charmer) - This suggests someone who is charming and persuasive, but it can also imply being insincere.
* Un séducteur (a seducer) - This is more explicitly suggestive of someone who uses their charm to manipulate or exploit others.
* Un manipulateur (a manipulator) - This is a more negative term, implying that the person is intentionally using charm to control others.
Idiom:
* Avoir la langue bien pendue (to have a tongue that hangs well) - This idiom literally means "to have a good tongue," and is a playful way to describe someone who is quick-witted and persuasive.
The best translation for "smooth talker" will depend on the specific context and the connotations you want to evoke.