The highest quality signal an unmodified N64 can output is S-Video, so I suggest you start by getting one.
Unfortunatelly, in the case of PAL N64, there are no readily available PROPER s-video cables.
NTSC N64's video and s-video outputs come out of the console at the right levels, so a cable with just wires inside does the job, but in the case of N64, the signals coming out of the multiout need some adjustement.
The composite cable has a resistor inside (which makes PAL and NTSC N64 video cables slighty uncompatible).
The s-video needs a similar adjustement, but, incomprehensibly, nor Nintendo or any third party ever made proper s-video cables for PAL N64, as far as I know.