Choosing host depends on budget really, and where your users reside.
If they are primarily from America,
choosing a host with server locations in Europe might not be the best choice.
For USA, while I usually prefer to go with cloud servers, I have positive experience with A2Hosting.
Depending on your technical skills, a VPS or cloud server (Hetzner, Digital Ocean, Vultr, etc) might be just as good and sometimes cheaper then a shared host. You would then have to arrange all backups, security and other server related tasks yourself. But if done correctly, it would provide you with a lot more flexibility.
For shared hosts, these are some things to consider:
The host should provide SSH access. (While pockethold can help some, I don't recommend it, even if I made it 😉 )
Let you configure PHP settings to some extent (IE enabling extensions usually disabled by default on many hosts).
Other then that, see requirements: https://docs.flarum.org/install.html#server-requirements this also goes for VPS.
I am sure you can get much more detailed recommendations if you provide a budget limit 🙂
