I have installed unity in my Ubuntu Gnome installation and using Unity Tweak Tool to customize.
What is the default window title font in Unity?
(I have changed everything else)
Below is a screen of "Ubuntu tweak" tool for font space settings.
Also, you may use below command to reset fonts back to its default.
gconftool-2 --unset /desktop/gnome/interface/document_font_name
gconftool-2 --unset /desktop/gnome/interface/font_name
gconftool-2 --unset /desktop/gnome/interface/monospace_font_name
Command Source: Default Window Title Font
Hope it helps!
If you mean the window title font it's Ubuntu Bold 11
If you use Myunity you can easily check and change