4th to 5th.
Yeah, there were some baby steps and some CD games in the 4th gen but when the 5th gen kicked off, the was a sense of awe that we'd never felt before. 5th to 6th was great, too. It's when things went from looking like blocks to looking pretty convincing. I can still play a few 6th gen games today and be impressed. 5th gen (and to a lesser extent, 3rd gen) I never want to experience, again. Even 90% of the "great" games have aged pretty badly.