When I was in a film school I made my bachelors thesis about this exact subject so I've become very familiar with these movies. I made it in 2012 so I'm not too familiar with the newer movies though, but I know something about them too.
The Seinfeld parodies are pretty good. They have done a lot of work to get it look and sound like the original show. It was legit funny at some points with some actual seinfeldish jokes in it. Too bad when the sex scenes start the style always changes completely. You know there's something wrong in porn when the non-sex scenes are the ones you want to watch and the sex scenes the ones you want to fast forward. That happens in surprisingly lot of porn parodies. It bugged me to no end to see a scene build up into a sex fantasy about some famous characters, but as soon as clothes get off, the camera gets into handheld mode and goes to wild angles and the cast are nowhere near the characters they should play.
The Scooby Doo parody is one of the rare ones where the cast at least try to stay in the characters during the scenes. Bobbi Starr as Velma did pretty fine job with that.
I remember the Married With Children parody having a few legit funny scenes too.