I've having been using Spark for past several months at work and my personal system. It is fast, and gets things done. I like its clean UI and minimalist approach. The smart inbox feature is also worth noting, which will surely improve over time.
If you use gmail, Inbox is the way to go. I have been using it since there early beta and I have not looked back sense. It effortlessly organizes your inbox into categories based on what you read and when, as well as sorting you trips based on the itinerary. Along with Reminders and a Todo list, it is truly a wonderful email experience.
Astro is currently the best email client out there for Gmail (and I believe 365) accounts. They are still in Beta and are near full featured. It allows to send later, snooze and set reminders. In addition to being a great standard client, it has built in AI that is only getting better as the days progress. It helps you quickly find action items in your email, set appointments and decide what is important. If you have macOS or iOS try it out.
I might get some backlash for this one but for me I like all the manual rules I can create because I'm an organization freak. Also looking for a tool that turns incoming emails to lead management...so until I find that I will be using excel and rules with Outlook :)