Hello,
PSX is the main system I like to play and I have tinkered with so many settings. The short answer is using custom resolutions and a scanline overlay.
In the end I setteled on the method in this thread,
which in my opinion gives the best visuals on a 1080p tv for all systems including PSX. There is also an awesome tv overlay that looks really nice, but also you have the option of not using it.