General:
* Readable
* Clear
* Distinct
* Plain
* Understandable
More specific:
* Decipherable (if it was difficult to read at first)
* Comprehensible (if the meaning is easily understood)
* Well-formed (if the writing itself is neat and organized)
* Easy to read
* Unmistakable
The best synonym depends on the specific situation and what you want to emphasize.