One feature? Native iOS Push E-Mail support.
This is also backed up by proper IMAP support for third-party e-mail clients.
You see, I'm not really a webmail user — I've gone through phases of working that way, but ultimately I still prefer to use the native macOS and iOS Mail apps on my devices. I rarely visit the FastMail web interface at all except in rare situations where I'm using somebody else's computer and need to write an e-mail of more than a few sentences (anything less I just do use my iPhone for).
FastMail was the very first provider to do native push e-mail support
properly (and may still be the
only one AFAIK). Ironically, even
Apple doesn't do proper push e-mail support with its own iCloud service (you get notifications of
new messages, but not any other status changes such as read, deleted, moved, etc).
To be fair, I was a FastMail user long before this feature came along, but I'd gone back and forth between FastMail and Gmail a couple of times before settling on running my own mail server (which I
wrote my own push notification code for). When FastMail brought full iOS push e-mail support, that pretty much sealed the deal, and I've been on FastMail and haven't looked back since.
However, even prior to that, the attraction to FastMail was also good support for open standards such as IMAP, and good server-side features for things like aliases, sieve rules, domains, and so forth. Ten years ago, I described FastMail to my colleagues as "everything I'd pretty much want to do with my own e-mail server." To be fair, Google has closed that gap for G Suite users, but Gmail is still a system that's better used via the web and with Google's own mobile apps (which don't provide as great of a user experience on the iPhone, IMHO). Google's implementation of IMAP has always been half-baked at best; on the surface there's way labels have to map to folders, but below that there have always been deeper technical issues as well.
That said, I were an Android user and/or lived on the web more, I'd probably still be using Gmail, but as somebody who is solidly in the Apple ecosystem, FastMail provides one of the best back-ends out there for iOS and macOS users.