I was wondering what a really good quality RCA cable would be
You can’t do better than: http://www.bluejeanscable.com/store/composite/index.htm
With that being said, the most important factor in getting a good LD picture is the player you’re using. LD video, as you know, is analog and not all players are created equal. An excellent composite video cable can help prevent factors outside of the player from having an impact on the picture quality, but it can’t improve the picture quality of a shitty player or a player that is out of alignment.
Also, LD video is stored on the disc in composite form. The s-video output on all LD players therefore uses a builtin (old and no longer top performing) comb filter IC. 99.9% of the time you’ll get better results with composite and an external comb filter, such as the one in your display.