Geary★★★★☆Really lightweight but still modern, so it’s a good choice on lower-end hardware. Basic IMAP features, good for most day-to-day email use.
Gmail (Android App)★★★☆☆Works well with multiple accounts and display modes, but tracks you more than it should.
K-9 Email★★★★☆Classic email app for Android: No frills, no ads, no tracking. Supports multiple accounts, phone-to-tablet layouts, and dark mode.
Microsoft Outlook (Android)★★★★☆It works. More stable than the desktop version. Handles mail, calendar and contacts, offers the focused/other inbox view. Tries to keep you in Microsoft’s apps. OK for work, wouldn’t use it for personal mail.
Nextcloud Calendar★★★★★Self-hosted, web-based calendar that syncs easily with other apps and has completely replaced Google Calendar for me.
OpenTasks★★★★★Simple to-do list that works great with a Nextcloud server.
Thunderbird (Email and Calendar)★★★★★Stable, capable desktop email application, works well with multiple accounts including Gmail, Nextcloud, easy to set up and use but with advanced settings when you need them. FLOSS.
Windows 10 Mail and Calendar★★★☆☆Not a bad email client. Snappy, works with multiple accounts. Some issues with Nextcloud calendar and contacts. Of course it’s being discontinued.