From seasons 1-12 (I'll do later the Jean era).
Two Cars In Every Garage And Three Eyes On Every Fish: It's kinda strange to choose this one because I love it, but it isn't really an episode that stands out in its season. Great, but not ultimate like some NHC users say.
Brush With Greatness: Same as Two Cars, but it's even worse from my point of view. Probably one of the five worst from its season.
A Streetcar Named Marge: I have discussed it several times. This episode from my point of view doesn't have good qualities over any other classic. It's of course solid and not boring but not one of my favorites and in fact a bottom5 for season 4.
I Love Lisa: It's good, but not easily remarkable in its season. I have always disliked the third act a bit.
Bart's Inner Child: One of my least favorite classics, an average episode for me that seems to receive too much love here.
The Last Temptation Of Homer: I don't really think it's overrated in the strict sense (it has my highest grade), but it's far overmentioned above some greater episodes in its seasons. Also, Colonel Homer was slightly better.
Bart Gets An Elephant: It's difficult for me to enjoy this episode at a great level. The humour is low and the storyline is too wacky for my interest.
Homer The Great: Again, I love this episode, but in my later reviews I have found it less interesting. It's not a matter of overall quality, but the fact that the jokes didn't work really well on reviews and they all seemed to get old.
The Springfield Connection: Another least favorite classic. It has good qualities in general but the plot development goes too far and makes it look like a fanscript IMO, where the characters hardly keep their usual attitude.
Team Homer: It's hard to choose an episode from season 7 here but I just think it's the only one way overrated here. This and A Fish Called Selma, but I guess I can see why people enjoy the last one in that level.
Realty Bites: One of the worst episodes from seasons 1-9 and the first solid sign of decline from my point of view.
Lard Of The Dance: For me, this episode is decent, good at best. Not one of the best offers from its season.
Sunday, Cruddy Sunday: It seems it got much more hate in the past than now, but in my opinion it's just one of the worst episodes I have ever seen, bottom20 if not 10.
They Saved Lisa's Brain: I just don't see the qualities of this average, almost bad episode.
Beyond Blunderdome: It is usually mentioned as one of the best season 11 episodes; IMO it's awful and one of the most boring experiences with The Simpsons. The plot is absolutely tasteless. The jokes, except for a bunch of good ones, were bad and predictable.
Guess Who's Coming To Criticize Dinner: It's good compared to the season standards, but not really a standout episode from my point of view. Just decent.
A Tale Of Two Springfields: It's in my bottom5, one of the crappiest episodes overall. And I didn't find it funny.
HOMR: The ultimate definition for overrating. A terribly bad show with awful characterizations and a shitty script. I agree there are good gags here and there but overall it sucks Emmy balls.
Hungry, Hungry Homer and Skinner's Sense Of Snow: Like GWCTCD. The fact that they are solid compared to their contemporaries doesn't imply they are good and remarkable out of their season.
Children Of A Lesser Clod: It has usually some appreciation because of its jokes but I actually found it unfunny and therefore lacking redeeming qualities.