The tool most people should add first, because it works on the writing you were going to do anyway. Grammarly sits in the browser, in Word, in Gmail and in most text boxes, catching the mistakes you stopped seeing and flagging sentences that read worse than you think they do. The free tier alone is worth installing, and it has grown well past grammar into tone, clarity and rewriting suggestions.
Its generative writing is competent rather than special, so it will not replace a proper drafting tool, and the suggestions can flatten a deliberate voice if you accept them all without thinking. Use it as an editor that questions you, not an authority. As the thing that quietly raises the floor on everything you send, nothing else is close.