Though the answers are pretty upto-date and workable for GNOME3, there is one caveat and that is even when with using gsettings reset ...
or gnome-tweak-tools' restore default button, you may not actually get the default fonts defined by the distribution. For example, Ubuntu (with Unity DE) comes with Ubuntu font as default interface font, but if you install another DE on top of this it can change this default settings.
The values for default fonts actually come from gschema files in /usr/share/glib-2.0/schemas
directory. When we install a desktop environment metapackage, it provides overrides file to change these defaults.
If this kind of things happen, you need to manually revert back the default by writing a custom gschema override file.
The override files for each GNOME variants
For each cases, create an override file with this name /usr/share/glib-2.0/schemas/99_default_font_settings.gschema.override
file and fill it with the following contents
Ubuntu with Unity
[org.gnome.desktop.interface]
font-name="Ubuntu 11"
monospace-font-name="Ubuntu Mono 13"
[org.gnome.desktop.wm.preferences]
titlebar-font='Ubuntu Bold 11'
Ubuntu with GNOME3
[org.gnome.desktop.interface]
font-name="Cantarell 11"
monospace-font-name="Ubuntu Mono 13"
[org.gnome.desktop.wm.preferences]
titlebar-font='Cantarell Bold 11'
Ubuntu MATE
[org.gnome.desktop.interface]
document-font-name='Ubuntu 11'
font-name='Ubuntu 11'
monospace-font-name='Ubuntu Mono 13'
[org.gnome.desktop.wm.preferences]
titlebar-font='Ubuntu Bold 11'
Once the file is saved, use this command to compile the schemas
sudo glib-compile-schemas /usr/share/glib-2.0/schemas
Then any of the above methods can be used to reset to the default set by you.
Alternatively, if you've installed GNOME environment on Ubuntu, removed ubuntu-gnome-default-settings
package to get back the ubuntu defaults. MATE's package is ubuntu-mate-default-settings
. If you happen to have Ubuntu GNOME as default and installed ubuntu unity, removed ubuntu-settings
package.
Hope this will be helpful to some extent.