Ubuntu is not default at any of the top three providers. I don't think Amazon EC2 and Azure Compute have a default per se, though Amazon does support and maint

Ubuntu is not default at any of the top three providers. I don't think Amazon EC2 and Azure Compute have a default per se, though Amazon does support and maintain Amazon Linux (in the RHEL/CentOS family) in a way it doesn't do with any other distribution. Google Compute Engine defaults to Debian. Of course, all three make it easy to use most of the major distros including Ubuntu (and Debian). Google's GCP support contracts will even support the images for all distros which they publish directly as well as certain partner-published images. (Disclosure: I previously worked at Google on the GCE team, including much of the work to launch Google's Debian images and collaborating with Canonical to validate and help launch their GCE Ubuntu images. I have not worked for Google since 2015 but am still involved in Debian's public cloud efforts in other capacities.)