I think people forget the later seasons because FOX moved it around so much that it just became hard to catch. You just made me remember that I've NEVER seen the series finale of Married With Children, but I do recall it going downhill in the last few seasons.
The Cosby Show in its last few years never got as bad as MWC.
Both shows in their prime were incredible for completely different reasons, but overall, I'd go with the Cosbys simply because it never fell off as bad as MWC.
The last three episodes of Married with Children are arguably the worst ones. One is about them being held hostage by some bytch Bud was prison pen pals with. The final two involve Kelly getting married.
The episodes featuring Seven are all pretty bad, especially the "Captain Blood" episode.
The shyt where bud was a movie director was a fukking joke.
I remember the episode where they retired Buck was pretty bad.
It wasn't even that the stuff they were doing was crazy because the Alien episode is on of the funniest. The show just got lazy, bland, and predictable.