I'll also offer that Bonanza got BETTER in its later run after Adam left. I caught up with the "lost episodes" on Encore and they best any of the early years.
The Beverly Hillbillies took its nosedive in the last 2 seasons which would've been 8 and 9. Actually, parts of season 8 were fairly close to the original spirit of previous seasons.
Dallas did get somewhat weaker as the show went on but there was still plenty to like in those last seasons. J.R. checking himself into a mental institution and unable to get out was revenge served well!
Daniel Boone's post-Mingo episodes weren't as good but there's something with S1 that...