Budget and features, really.
At its simplest all you need is a virtual machine (Digital Ocean and Linode provide only these, with AWS this would be an EC2 instance). But somewhere like Azure or AWS offer other services which you may want, depending on what you’re building (i.e. both of them provide Postgres as a dedicated service so you don’t need to manage that as part of your Discourse install).
If you’re pulling in a team to work on this it would probably be best to talk directly with them? They’ll understand the requirements best.