I've been a Fastmail user almost since Day 1, and a paying subscriber for around 15 years. Even though Gmail has now overtaken FM in terms of ease of use and features, there is one thing that keeps me coming back to FM for my personal email provider:
Unlimited email addresses.
Using the format <anyname.folder@username.fastmail.com>, I'm able to give each company I do business with an email that is completely unique to them, which then gets filed into its own folder. This has saved me numerous times when there have been hacks and data breaches, and my email gets sold to spam/scam artists, who begin flooding my inbox with scams. I know immediately which company gave away my information (since they are the only ones to have that email address), and it takes all of 10 seconds to create a rule to forever block any messages to that email.
It's a feature that is incredibly useful, and I'm surprised that more people don't use it.