I've tried countless email clients for Mac, and Polymail is where I've finally found home a month or two ago. I even finally signed up for 'Pro' a few days ago so I can have the extra read-receipt notifications. Highly recommend it.
Mac, Android, iOS client for Gmail and Office365. Free read tracking, snooze, send later, and the rest of the good stuff, and an AI assistant to handle VIPs, automatically sort mail, and remind you of important messages.
For me personally, the best email client is Hiri. I came across it some time ago, took the 14 day free trial and I found it very useful, so I’m still using it. It makes my job much easier now that I cam organize my emails in few clicks.