Here are some options depending on the context:
For a tool used to measure:
* Measuring stick
* Straightedge
* Scale
* Tape measure (for flexible measurements)
For a person who rules a country:
* Monarch
* Sovereign
* King/Queen (depending on the gender)
* Emperor/Empress (depending on the gender)
* Head of state
* Leader
For a person who controls or directs something:
* Boss
* Manager
* Director
* Administrator
* Chief
Please provide more context about what kind of "ruler" you're looking for a synonym for, and I can give you a more specific answer!