It really depends. If using strictly Google accounts, then Kiwi is an excellent GMail client and Mailplane is also a strong contender. Although my other Email clients (Outlook, Airmail, Spark) all support GMail, nothing does it quite so well as Kiwi.
I rely on Outlook 365 for business purposes and have had far too many issues with Apple Mail to use it in that environment. I keep returning to the Mac Outlook client for O 365 (and it also supports other services, such as Google). Having said that, Spark and Airmail are both really good alternatives which support O 365. I like them both and would definitely endorse Airmail as my top choice, however I do have a few more issues with Airmail than with Spark.
Someone mentioned security and privacy concerns with Spark. When I first heard of this concern, I researched it and found to have no foundation. I think Spark is perfectly okay with regard to the privacy of your data.
If not for my lingering concerns with the stability of Airmail, I would have switched long ago - however, at least for now, I’m sticking with Outlook for business and Kiwi for personal use and always on the lookout for the next great thing - (I’m intrigued by GyazMail, for example, but I have a feeling it’s not going to play nice with O 365).