I would go with Ubuntu. This is what Discourse recommends. The standard install guide (discourse/docs/INSTALL-cloud.md at main · discourse/discourse · GitHub) says that:
The default of the current supported LTS release of Ubuntu Server works fine. At minimum, a 64-bit Linux OS with a modern kernel version is required.
If you don’t feel comfortable doing it yourself, you can go with a Discourse hosted plan or with another Discourse host (e.g. Communiteq).