I do not know much about the default mail client on iOS. That being said, I’d imagine it’s more secure/privacy-friendly than a lot of 3rd party options. Though, you are potentially trusting another party with your emails. Whether or not Apple can collect/read those emails is, as far as I’m aware, unknown. I would not backup your emails to iCloud, as it’s not E2E encrypted. Even if the Mail client is secure, and privacy respecting, iCloud is not.
It should be noted that by using Apple’s client, you’re not giving more data to your email provider. For example, Google won’t get identifiable info from your device, as you’re not using the Gmail app.
As for the best apps, the two that come to mind would be ProtonMail, and Tutanota. This is because both of their apps use E2EE to their own servers, to retrieve your emails.
TL:DR
Here’s how I’d rank them:
- Proton Mail, and Tutanota’s own clients.
- Apple Mail
- Everything else.