Is it under-rated? Perhaps its marketing isn't as strong, but it's the "father" of a lot of distros today. Personally when I want a server distro, I still go with Debian. Simple and solid. No nonsense.
I think Debian and its children are so much nicer to use from a server perspective. RHEL and its derivatives have definitely improved over the years but it seemed like Debian had the right idea early on. Things like how they managed Apache modules and an out of the box emphasis on 'conf.d'-style configuration directories are just a couple of things that come to mind. The modular thinking lends itself well to learning because changes are easy to back out and automation because it's easier to compose configuration.
Personally, I feel like Ubuntu gets a lot more attention, perhaps more on the client-side. Sometimes, the foundational work Debian provides isn't fully acknowledged.
Yeah, Ubuntu probably is the best-marketed distro today. And I agree that Debian probably doesn't get the praise it deserves – IMO the nicest things about Ubuntu come from Debian.
