Flat remix is a pretty simple icon theme inspired on material design.
Ranked in these QuestionsQuestion Ranking
Pro Nice cohabitation with default icons
When an icon isn't provided by the theme, you cannot really spot it as this theme is quite close of the default with still a subtle and classy look and feel.
Pro Under GNU GPL3
Based copyleft license.
Pro Very large collection
This set contains icons for many file types that are ignored by other icon sets.
Pro Flat but with some depth
Con Under GNU GPL3
Limited use elsewhere.