Celebrim
Legend
Well, the Assassin is one of the very few PrC's which has ever caused me to consider breaking my rule of no PrC's in my campaign world. Unlike 95% of the classes out there, it is not merely a core class with more bonus feats disguised as class abilities. Unlike 95% of the PrC's, it is not merely 'lets make a class which is more specialized at X' - which inevitably leads to a class with 10 different bonus feats, munchkinization, and as much flavor as 'Skill Focus'. The flavor is interesting. The special abilities aren't over powering, and there is a reasonable chance of the class being used for multiple organizations and not merely occuring as a single enclave somewhere in the world.