I mostly only ever use the form: "This should be ... " with a possible explanation why. We're all mature enough to not take it personally, so I'm not going to beat around the bush by writing a lot of proza.
The imperative of the last example ("Do this") leaves no room for argument (because I might be wrong).
Either that or I ask for clarification of a piece of code: "Why did you do this instead of this?".
Like already stated by someone else; your approach really should depend on the team you're in. I'd take a less direct approach with junior devs who tend to be a bit more insecure (and stubborn).