Depends on what you want.
Boost has two geometry libraries,
boost::geometry which is not robust but has advanced stuff like non-euclidean geometry (such as latitude/longitude on the Earth) and
boost::polygon which is robust but doesn't have as many features.
CGAL is another popular one, which is also very robust but can sometimes be very slow. CGAL has a lot of algorithms but its heavy template style may not be considered "simple" depending on your use-case. Furthermore, some parts of CGAL use the more strict GPL license (as opposed to LGPL), which some people may find problematic for a library. There is a commercial version you can pay for to work around those limitations.
Wykobi is less popular but still very good. It is not robust.
Robust libraries always provide things like topological guarantees, exact predicates etc. Non-robust libraries are usually faster but may produce geometrically or topologically incorrect results in some situations.