This is a good one. The Flyers scored 55% of the goals in the series, also took 57% of the shots, but still lost.
Game 1 was rough (nothing egregious by the standards of the Dead Puck Era, but lots of hitting and borderline dirty antics). The Flyers outplayed the Leafs and won 3-0. The Flyers again outplayed the Leafs in game 2, but Toronto scored two goals in the final two minutes to steal a 2-1 win.
Back in Philadelphia for game 3, the Leafs were badly outplayed, but Joseph was stellar (stopping 41 of 42 shots). The Flyers controlled game 4 (Joseph faced 40 shots this time, but looked human). The series was now tied 2-2, even though Philadelphia probably outplayed Toronto in all four games.
Game 5 was a close, even match. It was a goaltending duo, and Yanic Perreault scored the OT winner for Toronto.
Game 6 was uneven. The Flyers dominated the first period; you could sense their desperation, and perhaps their frustration. The second period was more or less even (both teams spent a considerable part of the frame on the powerplay - by my rough calculation, less than eight minutes of that period were at even strength). The third period was tense and close. Sergei Berezin scored the series winning goal with exactly one minute left in regulation.
Looking at the series as a whole, the Flyers outplayed the Leafs in each of the first four games (and the last two were essentially toss-ups). The Leafs outscored the Flyers in just 3 of the 18 regulation periods! I recall hearing at the time (but I don't have a specific source) that Toronto scored the fewest goals in NHL history (nine) while winning a best-of-seven series.