Geometry anti-aliasing, as its name suggest, only smooth out the edges of object (16x16 oversampling). The ray-traced surfaces of the scene (reflections, transparencies and alpha) are not anti-aliased.
Best anti-aliasing smooths out all surfaces.
If Geometry AA produces good results for the other elements of your scene it's not necessary to use a full Best AA as it will increase the rendering time dramatically. Instead use Best AA only for ray-traced surfaces (a reflective floor, a transparent curtain or an alpha mapped plane).
To do this set the Best AA sampling to Min1x1/Max1x1 (this is exactly the same as Geometry AA) and place a "force anti-aliasing" compositing tag on your floor object (Min2x2/Max4x4 should be enough). Cinema 4D will then use stronger AA for the floor only.
[C4D] Geometry or Best?