I used ProtonMail a few years back, it was kind of “spartan”.

I had heard good things about Fastmail from other GMail switchers and signed up for their free trial account, and liked the experience.

The mobile app is great, they support 2FA with hardware security keys, and the price seemed reasonable.

I didn't do a lot of shopping around, but I chose them because they appeared to have a good customer service, good pricing for single-person use, and all the features you might want like support for your own domain name, email aliases, caldav, etc. I have been a happy customer for years and all of these things turned out to be true. I have had zero reason to even consider looking elsewhere.

Also the web app is really good. Like really really good. And I don't normally like web apps.

Been using them for, I think, 16+ years at this point. Their support is great. Their product is great. The price is great. For a techie like me, it's perfect. I don't know if I would put Grandma on it, though, but perhaps I would.

Support is helpful and responsive in the rare edge cases I have run into twice over many years when pushing their features to extremes