Formal:
* Playwright
* Screenwriter (if the dramatist writes for film or television)
* Librettist (if the dramatist writes the text for an opera or musical)
* Scenarist (a less common term for a screenwriter)
* Dramaturg (focuses on the development and analysis of plays, not necessarily writing them)
Informal:
* Play writer
* Scriptwriter
* Writer (if the context is clear)
Other:
* Author of plays
* Creator of dramatic works
* Master of dramatic art (more poetic)
The best option for you will depend on the specific situation and the desired tone.