The most important question on a PR checklist is...

Admit it, you thought I  was going to say something about Testing. Didn't you?

While testing is super important and should be part of every PR, the most important question to ask when working on something as part of a team is:

Does this change include anything you need to communicate to the rest of the team?

I've added this to PR templates and have been frustrated when "circumstances" have prevented me from doing so.

Why does this matter?

• Because if it's something low-level or fundamental, then everyone is likely to need to know it. You shouldn't rely on everyone reviewing the PR or checking everything that gets merged. 
• Because the change(s) in the PR might break the code other team members are working on.
• Because a "team" implies working together towards a common goal. Keeping secrets and not sharing things that "teammates" will benefit from knowing hurts the team and the project.

Not sharing important knowledge and information creates frustration, resentment, wasted effort, and more.