According to some online sources, the Genesis is NOT s-video compatible. It DOES output RGB...you just need to build a cable and get an analog RGB monitor (with a horizontal scan rate of 15 khz), which haven't been around at least 10 years...TVs now have COMPONENT video inputs, for DVD players, which is NOT RGB!!!
Games X covers this:
http://www.gamesx.com/rgbadd/
Redmond Cable makes custom RGB cables for Genesis 1 and 2 for various plugs:
http://www.connectworld.net/rcc/pages/game.html
If you have the A/V cable for the Genesis, you could use a Video RCA to s-video adaptor--it splits the video signals into chroma and luma:
http://www.trianglecables.com/1insvidsvhst.html
But I don't know if it works well or not. Anyone know?

So it seems the only options for most consoles are A/V and s-video if possible...and if you're willing to pay for the custom cables and find a compatible monitor, RGB (if it outputs it--N64 does NOT, SNES DOES, but Redmond has trouble getting the parts to make RGB cables for SNES).