Greybird is the default theme of Xubuntu. The new version of the theme (3.20.0) now supports GTK 3.20 as well and it has been ported to SASS.
Linux Mint 18 is going to introduce a new GTK and icon theme called Mint-y. The GTK theme is based on Arc and the icon theme is based on Moka.
Adapta is a shiny new material design like GTK theme. It supports numerous desktop environments including Gnome, Budgie, Unity and Cinnamon.