The best VPS OS is the one that best suits your needs. What mainly makes any OS awesome is the support quality it offers.
Debian is a Linux-based OS which is very popular. However, it differs a lot from Red Hat Enterprise Linux operating system (RHEL) based systems, though is shares certain management features common in all Linux systems. Debian shares conservative approach with CentOS, yet update packages are applied much more often and their number is significantly higher. Compatibility with a high amount of software is much wider due to its constantly updated packages. It is a stable too.
Ubuntu resembles Debian a lot. Its orientation is more on desktop or graphic interface solutions. It still can be used as a decent server OS but not very popular out there.
Fedora is also RHEL based system. It is developed fast, and updates are often released. The OS management differs much in the different Fedora versions available.
CentOS is another RHEL-based systems. This means some Fedora operatives, like EPEL, contain packages compatible with CentOS. The difference between the two makes some control panels and software incompatible between CentOS and Fedora.
I hope this information will help you make a more informed decision. Thank you.