I'd recommend git-spice, it's very easy to use and powerful, and of course open-source. I've tried graphite but found that they made it too convoluted for what it is.
git-spice is specifically targeted to be useful for PR stacking and doesn't require you to do anything differently from normal git operations that you likely use already. It has a bunch of really nice flows and doesn't try to step outside the bounds of what is needed to easily stack PRs