The following default usernames are listed on https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connection-prereqs.html#connection-prereqs-get-info-about-instance:
- Amazon Linux 2 or Amazon Linux AMI:
ec2-user - CentOS AMI:
centosorec2-user - Debian AMI:
admin - Fedora AMI:
fedoraorec2-user - RHEL AMI:
ec2-userorroot - SUSE AMI:
ec2-userorroot - Ubuntu AMI:
ubuntu - Oracle AMI:
ec2-user - Bitnami AMI:
bitnami - For anything else, just hope you can find it by duckduckgo-ing