I would compare Diesel and Energie to D&G. Italian brands with unusual cuts and styles. Diesel was one of my favourite brands when I was younger, but then the clothes became extremer and the prices went up. Diesel and Energie are now a little bit too "loud" for me.
Stussy is streetwear at it's best. A little bit skatestyle combined with different musical influences (Rock, HipHop etc.) but without sloppy baggy pants and fake gangster-appeal. Plus, the quality is always great (they still produce most of their stuff in the USA) and not everyone's wearing it (at least here in Germany very few stores sell the clothes).