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.)