For me. . . .
Seasons 1-4: Gold standard, definitely when the show was in its prime.
Season 5: Despite some of the original key creative people still involved, Arthur's voice drop was disorienting.
Season 6: My PBS skipped this season altogether for the longest time, but once I did finally see it (via On Demand in 2004), it was incredibly weak.
Seasons 7-8: Definitely an improvement over the previous couple of seasons with a number of memorable episodes.
Seasons 9-15: Weak, unbelievable stories and miscast voice actors, never bothered.
Seasons 16- 18 1/2: I mean yeah, the Flash animation was definitely off-putting at first, and the Compsons were very polarizing, but I wouldn't say these were either bad or good . . . plus, the return of one of the original writers from Seasons 1-4 definitely brought back a sense of that classic ARTHUR feel.
Seasons 18 1/2 - Present: Just grew weaker and weaker, and like Any said, don't care anymore.