Ask HN: What tone to use in code review suggestions?

When writing suggestions in code reviews I have used all of these forms:

* Should we extract this to a separate function?

* Could you extract this to a separate function?

* I would extract this to a separate function.

* This could be extracted to a separate function.

* This should be extracted to a separate function.

* Extract this to a separate function.

As you can see, these have very different tones and I would like to be more consistent and as constructive as possible. Is there some general best practice for this? Are you or your team using a set of rules or guidelines?