Microsoft Outlook is the best Windows calendar app for most people, offering a native Windows interface, combining email, calendar, contact management, and task list features, and syncing with Microsoft 365 and Exchange calendars. OneCalendar is the best option for those who want to connect their calendar to every platform on earth, including Google Calendar, iCloud, CalDAV, WebCal, Yahoo, GMX, mailbox.org, Nextcloud, Synology, and ownCloud. Mozilla Thunderbird is a solid open-source alternative that offers flexibility through community-built extensions, but may have a cluttered main view. For those who don't love any of the above options, there are other alternatives like Rainlendar, which adds calendar widgets to the desktop, or using Google Calendar in a browser.