I would argue 4, with the addition of Castiel, is where it really hits its stride as a scooby-doo type monster of the week beat with a full cast outside of the family to cut through some of the melodrama. But that's the hard cut-off for me because after that's when they start dying and bringing each other back from hell over and over again and their sibling drama is super played out at this point etc. They could have done something interesting 7 and onward, but they didn't. IDK I stopped at the end of 7, except to catch the Scooby-Doo crossover episode which was alot of fun.