My personal favorite filter/shader combination is cgwg’s CRT shader together with blargg’s NTSC-RGB filter. Unfortunately, the available SNES version for RetroArch of the latter doesn’t work for cores with a different resolution, e.g. the GenPlus GX core. Compiling the core with the NTSC flag did not produce the result I wished for and resulted in scaling errors for some ROMs (e.g. the Genesis version of Addam’s Family).
To get the result I wanted, I ripped out the NTSC filters from the original GenPlus GX project (based on blargg’s library and modified by EkeEke) and created a set of separate filter DLLs for the monochrome, svideo, composite and rgb variations.
You can find the source code and compiled 32 and 64 bit binaries for Windows on the GitHub repository I created for it. I hope these are useful for others looking for Genesis and Master System filters.