Hey, Tuta Team here. If you're looking for one with unlimited addresses for a custom domain, check out Tuta Mail: https://tuta.com

What is best depends on one's means, priorities and preferences at a particular time. You'll get better answers if you shed some light on those.

Fastmail is the best I have used. There may be better that I haven't.

Mailbox.org ? (Haven’t tried it yet but saw some good reviews

Dreamhost