Advanced Formatting in Mail
It may seem like a small thing, but Apple’s built-in Mail app now offers a full desktop-class text formatting experience. This isn’t just bold and underline and font colors and sizes either. You can actually use any font installed on your devices (and iOS 13 now more easily supports adding custom fonts), to create very richly-formatted emails. There’s so much here that it takes up an entire palette of controls that replaces the keyboard when in formatting mode.
Mail also now lets you set colored flags to help categorize your emails — something that will surely delight users of Apple Mail on macOS, which has supported this feature for years. So if you add a green flag to an email on your Mac, it will show up with a green flag on your iPhone now too.