TV-Tuner/75-Ohm coax is the SHITTIEST video quality you can get. They are only made for very old televisions which do not have RCA inputs (very old as in made before 1986). Never use one of these unless you absultely have to. The reason it looks so bad is because it carries all video and audio data on one wire and results in everything picking up noise since the tuner has to filer it all back out to separate signals.
RCA cables are much nicer. It carries composite video on one cord and left/right audio on separate ones. However it is still mixing R, G, B, V-sync, Sync, and Ground on that video line.
S-Video uses luminence and chrominence. It's a totally different kind of video signal which was not around till recently. It's quite nice for the price if your TV supports it.
Straight RGB is the best way to go. Each line carries R, G, and B separatly with video sync. It's the cleanest there is next to VGA.
If you live in Europe, you may want to look into a SCART cable since that would be your cleanest picture (as I recall SCART is a DB-25 conection and carries all the signals on different pins).