General:
* Proficient writer
* Accomplished writer
* Talented writer
* Expert writer
* Masterful writer
More Specific:
* Craftsman/woman of the written word
* Literary artist
* Wordsmith
* Penman/woman (for historical or formal contexts)
* Author (if they've published works)
* Journalist (if they specialize in journalism)
* Copywriter (if they specialize in advertising)
Figurative:
* Master of the pen
* Weaver of words
* Sculptor of language
The best word to use will depend on the specific context and the tone you're aiming for.