Codeforces Expert, CodeChef 5 star. The ratings are old news; the habit they built is not.
Not the ratings. The reflex of reading the constraints before writing a line, and of asking what breaks at the limit rather than what works in the demo. Years later that is still the most useful thing I carry into a backend problem: the slow query, the race between two requests, the input nobody thought to try.
After a few years of competing I started writing the problems instead, 500+ of them for Masai and LevelUp. Setting a good problem is harder than solving one. You have to find every edge case before a few thousand people find it for you, and you have to make something hard feel learnable.
Honestly: I do not grind contests any more, and the ratings have not moved in a while. I keep this here because it is where the fundamentals came from, and because it is a hunger I would like back one day.