GitHub Actions hands down. Huge amount of community-provided actions, easy to get started, lots of ways to reuse workflows or parts of workflows. Perfect integration with GitHub obviously.
You get deep integration that is hard (at least for me historically) to get anywhere else.
I have a monorepo, with builds and deploys for each directory, so if I make changes to only 1 of my n-many directories, it only builds and deploys for that 1.
But with things like Vercel, Netlify, Render, Heroku (rip lol), and others having built in CI, it leads to interesting conversations around centralized CI (GitHub Actions) vs decentralized CI (aforementioned vendors).
I hadn't realized the difference between the two before now, but they have advantages and disadvantages.