Ubuntu Server 18.04 LTS without any additional software. SmartAMI upgrades it periodically so it includes newest available packages. You will be notified when the new version is published. Use of an up-to-dated version of Ubuntu Server 18.04 LTS Bionic Beaver along with its packages makes your life easier and your deployments faster. SmartAMI engineers test it before it is published to ensure that it is free of problems with dependencies and that it runs in AWS environment without any issues.