The richness of colour makes ROTJ the nicest to look at but it lacks real depth for me. ESB is the best directed and acted but it's the most dated costume wise with loads of 1980's influenced design like theHoth costumes and the Bespin designs and colour scheme.
ANH is my favourite. I went into some detail on this in the old forums and I'm too tired to go look for it but ANH is the original that inspired my lifetimes achievements and aspirations. It's the most honest of all three, the most innovative. The others are just rehashes of already used ideas with minor surprises thrown in like halloween candythat isn't really candy.... Not really the best analogy but I'm sure you take my meaning.