Which API should I use to list all environments?

In order to list all the available environments you can use the EnvironmentList endpoint:

https://app.valohai.com/api/v0/environments/

This will list all the environments the API token creator has access to. If you want to restrict the list to a certain project, you can use the project=<project-id> query parameter: 

https://app.valohai.com/api/v0/environments/?project=<project-id>

The project id is available under the project Settings.