To me it would be Gen 6 (PS2 / Xbox / GC) as it was not only the golden age of 3D platformers, but the developers were not afraid to regularly introduce new gameplay mechanics that were specific to a single level. Very few games do that nowadays, the most recent examples I can think of are Psychonauts 2, It Takes Two and Split Fiction.
To me it would be Gen 6 (PS2 / Xbox / GC) as it was not only the golden age of 3D platformers, but the developers were not afraid to regularly introduce new gameplay mechanics that were specific to a single level. Very few games do that nowadays, the most recent examples I can think of are Psychonauts 2, It Takes Two and Split Fiction.