core-js is the more common of the two in our index: it runs on 15,005 websites against 20 for Ramda, and 19 sites use both. Counts come from real scans, not vendor claims.
core-js is a modular standard library for JavaScript, with polyfills for cutting-edge ECMAScript features.
Ramda is a practical functional programming library for JavaScript that emphasizes immutability, currying, and function composition to facilitate writing more declarative and modular code.
19 sites run both core-js and Ramda. That is 95% of the sites using Ramda, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.