What do you call a person who always follows the rules, at the expense of everything else? I’m thinking there’s one word that can describe this, but I can’t place it.
A jobsworth is “someone who always obeys the rules of their job exactly, even when it would be more sensible not to”.
The Free Dictionary Online would say that person is:
A person inflexibly attached to a practice or theory without regard to its practicality.
This is also the adjective form. An alternative noun form is doctrinarian.
Also, in Biblical usage, a Pharisee.
I’ve encountered a few people that you describe. Often, they were bureaucrats:
An official who is rigidly devoted to the details of administrative procedure.¹
If we focus on the “at the expense of everything else” part of your question, both fanatic and the previously mentioned zealot are good options. Adjectival variations include fanatical rule-follower, a fanatic for X, etc.
From a different perspective, a martinet is a strict rule-follower, but the term is less used nowadays. Martinet has a pseudo-military connotation and draws focus to making other people follow the rules.
If you were using this term in a gaming context, that person would be known as a "rules lawyer".
You could say such a person is lawful.
Conforming to, permitted by, or recognised by law or rules.
Not sure of the original question, but id say that the best word for "rule follower, " is bureaucrat. God dammed bureaucrat. No skilled, bastards. Live in stale cubicles, starring at a computer all day, smelling someones leftover dinner being reheated in the break room microwave. Dreaming up rules for others to follow. Can you tell I'm self employed?