That’s a very broad question that depends a lot on your usage. My needs may be different from yours.
I’m currently using Migadu because:
Unlimited domains, mailboxes, accounts and aliases for a flat fee. I’m managing accounts for myself as well as family and extended family members and it comes out much cheaper this way than services that ask $5-10/account.
Very nice management interface with all the bells and whistles but with reasonable defaults and easy to use.
The company is based in Switzerland and the mail hosted in EU (France).
Standard email service with everything you’d expect (the regular protocols, spam protection, webmail, full compatibility with clients etc.)