As a former Simpsons obsessive (still watch if it's on!) I would say series 3-10 kind of area were the best episodes, with the most memorable storylines.
If anything I would say it's paid for the higher quality animation in later series.
I can't remember the last time I watched a new episode and actually thought it was any good.
Can't quite put my finger on why but Flaming Moe's has always stood out for me as a great episode!