Hi! Recently I watched first two seasons (ABC season and Syndication season) of the Real Ghostbusters and I’m gonna say, this cartoon was really ahead of it’s times. It has really impressive writing and characterization, probablt the best cartoon of the 80s. But, I read that after first 78 episode, show gone downhill because of really dumb changes demanded by network. It’s a shame that show got treatment like that. But I refuse to believe that every episode after original 78 ones sucks. What are those diamonds in the rough in later seasons in your opinion?
Oh there are some definate diamonds in there. There is just a dip in quality when compared to first 2 seasons. The season that suffered the most was the final season, but even that one has a pretty nice diamond. Just because there was a dip in quality doesn’t mean the series wasn’t still entertaining. Since you made through the best of the series I don’t think you will have a problem getting through the rest, but here are some episodes not to skip.
The Boogyman is Back
The Grundle
Halloween 2 1/2
The Two Faces of Slimer. ( I just love it when Peter gets slimed and this one is heavy on that.)
Poultrygeist (This is a silly episode, but I have a real soft spot for it.)
Robo Buster
Season 4 is pretty short so It’s easy to get through.
Loose Screws
Somethings Going Around
Jailbusters (This one has some very choppy animation and you’ll notice that the episode is slowed down to fit the hour format of the show at the time, but I enjoy it when they go into the spirit world.)
Ghostbusters Live From Al Capone’s Tomb
Elementary My Dear Winston. (This is another one that has some odd animation and timing issues, but still fun.)
The Halloween Door (This one aired during prime time so they were able to get away with things they couldn’t do on Saturday Mornings. It also has some of the best animation in the series. Definately the best episode to air after Studio Interference.)
Season 6 and 7
Janine You’ve Changed
Not Now Slimer
Skip Deja Boo it’s a Slimer focused episode and it is a clip show of better episodes. Unless you really enjoy Slimer and Dweeb.
I hope you enjoy the rest of the series. Definately don’t miss the episodes above. 
Thank you for such elaborate answer! 
I’d add a few more
Big Trouble with Little Slimer (return of Peck)
The Brooklyn Triangle (Winston’s dad, the Collector wasn’t a villain after all)
The Haunting of Heck House (taking on a very haunted house with no Proton Packs)
The Treasure of Sierra Tamale (international bust with new tech, Peter’s dad, a god)
Follow that Hearse - Spooky, a little scary in points, with great atmosphere… and Ecto-1 being possessed and mutating.
Thank you all of you for recommendations!
I actually finished season 3 in full now, it isn’t was too shabby, I’d say. Some changes are noticeable, but quality is still pretty high for the most part.
Ash vs Evil Dead did something similar in its second season with the classic.
Ive mentioned that while there was definite dip in quality in the later seasons (story editing wise). There were definetly some great episodes that I do love.
My list.
The Boogyman is Back
The Grundle
Halloween 2 1/2
Robo Buster
Loose Screws
Janine you’ve Changed
Ghost World
The Slob (the only reason I like this episode is because after all those lame Slimer cartoons it was damn cathartic seeing the GB’s tell professor Dweeb off…seriously as a kid I freakin cheered.)
I agree with all of above lists but I can’t help but notice “Halloween 2 1/2” and the “Boogyman is back” being mentioned alot.
I know I’m in the minority but I feel those two episodes have the most negative aspects of what fans disliked from the series.
Samhain and the Boogieman are my two favorite ghosts in the entire series but those two episodes are the start of the Q5 take over and I can’t really watch them.
“The grundel” was a excellent episode and I enjoyed all of the mentioned episodes but to me the beginning of season 3 until we get to the grundel was bad.
The Haunting of Heck House and It’s About Time are my favorites 
You’re not alone. I’m on the fence about them, but there’s both good and bad to them.
there was alot lol mostly during the peter best friends with slimer era