Try to avoid the cheapest scarts, as they are prone to slight feedback from TV out returning back the input lines, leaving an unpleasant moving 'ghost' of whatever channel the TV was last tuned to overlaying the DVD signal.
Its not always visible, but in 'space' movies, or really anything with a lot of black it can become very obvious. Its reduced in RGB mode even with cheap scarts, but a decent cable has clearly visible advantages.