According to the Ask Ubuntu Help Center (which I think you were prompted to look at when you signed up ;-) ), the following distributions are considered official Ubuntu flavors:
- Kubuntu
- Lubuntu
- Ubuntu Budgie
- Ubuntu Kylin
- Ubuntu MATE
- Ubuntu Studio
- Xubuntu
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 Ask Ubuntu 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, Ask Ubuntu won't have it immediately.
