My favorite episodes are:
Any episode from seasons 1-5
"For Whom The Bell Tolls" and "Brother,Can You Spare A Clarinet" from Season 6
Any episode from seasons 7 and 8
"Francine's Big Top Trouble","Arthur Makes Waves",and "Binky Goes Nuts" from season 9
"Happy Anniversary" from season 10
"The Making Of Arthur" from season 11
None from season 12
"The Great MacGrady" from season 13
None from seasons 14 and 15
"So Funny I Forgot To Laugh/The Best Day Ever" from season 16