Which which-key should I install?

which-key 3.6.0 available gnu Display available keybindings in popup which-key 20220214.1818 available melpa Display available keybindings in popup

You are good with either of those. It is the same version, just named differently. The Melpa versioning scheme is a bit unfortunate, since it does not use the same semantic versioning scheme as rest of the community but uses dates when package is submitted. But it is built on 2022, 2nd Feb, which is the day of the latest update when which-key was bumped to version 3.6.0. You can check [the GitHub repo](https://github.com/justbur/emacs-which-key).

The version on melpa is whatever's currently on [the Github repo](https://github.com/justbur/emacs-which-key). The version on GNU only updates when the author actually publishes a release. So depends on whether you want to live on the bleeding edge or not.

Thanks!