This C64 fanboy has to admit that the CPC version looks the best. There may be only 4 colours on-screen at any time but it's surprising how little difference this makes once you get used to it. The better definition on the characters, compared to the C64 version, is worth the loss of some variety in the colours. At least with the CPC you can clearly see what you're picking up, unlike the blocky multicolour sprites used for the C64 version - you can barely make out what some of the objects are.
The CPC version runs a little slower than the C64 version but in many ways that's a bit of a relief! Speed isn't really an issue with this game. The Spectrum version I haven't played, but it looks fairly bland. The backgrounds look unrelentingly monochrome - I guess it would've been too confusing to have different colours in the background when there are lots of characters moving across in front of the backdrops.
Soon you will have forgotten all things: soon all things will have forgotten you. (Marcus Aurelius, Meditations, Book 7)