you can use an original PS2 rgb/scart cable if your tv has a scart output and make sure that you don't confuse scart input with DVI. the rgb/scart cable is the best that there is for enhancing PS1 picture quality on an hdtv.
your second option would be to use s-video. you can either use an original PS1, PS2, or the newer PS3 s-video cable. PS2 or PS3 component will not enhance picture quality as it will mutually be on par with s-video. the latest s-video cable at major retailing stores that you can use is the PS3 cable. the PS3 s-video cable is compatible with all PS consoles (fits every console video input) and retails for $10 bucks.
you can grab an rgb/scart cable on ebay.
PS1/SCPH-7501 | MM3 modchip
PS2 v9 | 250GB hdd | PS3 component cable | katana 32mb mc
PS3/CECH-2001B | 250GB hdd | 3.65 ofw