I personnally think Sun Tzu is a classic...heck, it's a materpeice. Unlike Machiaveli or Clausewitz, this text is still very relevent. It focusses on strategic perspective and analysis that are common to any conflict analysis. Furthermore, the text is much more focussed on attaining your objectives WITHOUT actually initiating combat : espionnage, sabotage, bribery, tradecraft and indirect warfare. Personnaly, I always tough that direct military confrontation is to be used only as a last resort. Military conflict is a waste of precious ressources (including $$$, human lives and diplomatic ressources) and should not be considered an end by itself. Indeed, it should be avoided up until the point where diplomacy and indirect warfare fail.
I think the analytical framework and paradigms presented in the book are still relevant today, even tough most armies don't fight with archers or pikemen anymore!