That window is too narrow. You're leaving out some amazing episodes, ie Bart Sells His Soul (7), Marge Be Not Proud (7), You Only Move Twice (8), Homer vs the Eighteenth Amendment (8), Homer's Enemy (8)--even season 9 has some good ones, although I agree that's where it drops off--The City of New York vs Homer Simpson (9), This Little Wiggy (9).
The show really hit its stride in season 4, the season which contains my all-time favorite episode Last Exit to Springfield.
I agree - how can one omit seasons 7 and 8 from the show's peak?
Bart Sells His Soul is definitely one of the best episodes of all time. Season 7 also has great episodes such as Team Homer, Two Bad neighbors, Homer the Smithers, and many other solid episodes.
Season 8 is even better, with classics such as
You Only move twice
Hurricane Neddy
The Mysterious Voyage of Homer
The Springfield Files
The Twisted World of Marge Simpson
Mountain of Madness
Homer Vs. The Eighteenth Amendment
The Canine Mutiny
Homer's Enemy
The Secret War of Lisa Simpson
And though The show started to slip a bit in Season 9, the early part of the season actually has some of the show's greatest episodes.
The City of New York vs. Homer Simpson and
The Cartridge Family
match up against any episodes from any season.
Bary Star and Das Bus are strong episodes. This little Wiggy and Simpson Tide are strong episodes. Same goes for The Trouble with Trillions, Girly Edition, and Trash of the titans.
I will admit that the other episodes from season 9 are decidedly weak, and that from season 10 on there are few gems to speak of.