What is the best operating system for a server? Arguably it comes down to either CentOS or Ubuntu.
The good point about Ubuntu is that it has a much larger repository than CentOS and so many more applications are available by default.
Also Ubuntu has an AMI image repository of pre-prepared, blank and safe images that can be easily launched. Consistant In every geographic area.
Simple point and click and the server will be started in the current AWS Console.
Remember to login as the user ubuntu.