I've learned that commands not phrased as such usually aggravate people the most - especially #1 from your list or questions where the one questioning knows the answer and wants to suggest something.

If I don't know how to phrase my comments I just schedule a call and we go over the code together. Much less time spent than on comment ping-pong.

If the code generally works, but other aspects, like maintainability, would benefit from some rework I use the phrase "You can...", e.g. "The second argument to the `map` callback is the element index. You can use it to create this range instead of using an external variable incremented on each step."

The recipient has the possibility, but is not actually forced to do anything, just like with code suggestions. In the vast majority of cases they follow my advice though.

If there's an error or a kludge I use:

-Code suggestions - no comment, so nothing to get upset over and if the recipient is feeling particularly irritable, they can just ignore it.

-Direct language in imperative form, e.g. "make sure that X, otherwise this won't work as intended", "avoid X in Y", "use Z here (documentation link)".