According to the [Ask Ubuntu Help Center](http://askubuntu.com/help) (which I think you were prompted to look at when you signed up __;-)__ ), the following distributions are considered official Ubuntu flavors:

- Edubuntu
- Ubuntu GNOME
- Kubuntu
- Ubuntu Kylin
- Lubuntu
- Mythbuntu
- Ubuntu Studio
- Xubuntu
- Ubuntu MATE
- Ubuntu Touch
- Ubuntu "Snappy" Core

All of these are on-topic. Honestly, there isn't much difference among these. It's mostly just the desktop environment, themes and certain pre-installed packages.

Since AskUbuntu tends to reflect the list of official flavors on the Ubuntu website, you may also want to look there. Chances are, if a new official flavor is named, AskUbuntu won't have it immediately.   
https://wiki.ubuntu.com/UbuntuFlavors