I like simple apps that do their job. This is really lame, but my favorite app is officially the Windows 10 Mail app. It is just so simple. It lets me connect to many email accounts in one place. It looks really good on the screen. The Reading Pane makes plain-old-email look so much nicer than Outlook. It’s also not a hog like Outlook. I hate Outlook now and when I cannot use Windows 10 Mail, I just use web access for O365.
Best of all is that the Calendar part (which may very well be another separate app, but I consider it all the same) lets me pull Google and O365 calendars into the same place. All color-coded. Alerts come in from all of them in my taskbar, just like one would expect.