I use a PS3 hooked up through HDMI to watch dvds on my hdtv and although HDMI looks great for hi-def material it doesn't look so great playing standard-def dvds on my HDTV, it really shows their age.

can someone on here tell me what cable can be used to get the best quality out standard-def dvds played on my HD television?

You're already using the best option. Your PS3 can only upconvert a DVD via the HDMI cable. Check your settings in the PS3 to make sure everything is set properly. Do you have another DVD player you could connect to your TV to compare PQ with the same disc? Keep in mind that all DVDs are not created equal, PQ can vary from disc to disc. How many DVDs have you tried playing?