I recently completed this question as part of a job application and thought I’d share my thoughts.
Inbox by Google
On the surface just another email client, right? But no, there’s a lot of non obvious niceties that save time and reduce your time in the app so you can go about your day. Gestures are one of the key elements in Inbox’s design that I love. Swiping down to close an email just feels nature and makes using one handed effortless. Also from the inbox list Swiping emails or bundles in left/right to either dismiss or mark for reminder.
I heavily rely on pinning messages that require further thought/actions and so being able to switch to show only pinned messages straight from the app bar saves me digging through menus.
The dynamic quick responses are often spookily accurate to my replies, which again saves time. Another time saver the dynamic recent contacts list (with profile photo for quick recognition) appear in the FAB sub menu.
I haven’t achieved a regular inbox zero, but I did once and instead of just a empty list Inbox shows a lovely full page happy graphic which feels like a reward/achievement in a game.
In a nutshell the niceties and time saving subtle features turn a mundane email app into something very special.