They were good; no doubt. As a Flyers fan, I really disliked the Islanders, as the two teams battled a great deal (including the SC Final in 1980) over the span of the Islanders dominant run.
I would not anoint the Islands the best dynasty team ever, even within the NHL. They won 4 straight Stanley Cups, and lost in the SC Final the following year. That's certainly impressive. I think the Oilers of the 80s were a better dynasty They won 6 SCs in 7 years, and appeared in the SC Finals 7 times in 8 years during that span. Before the Islands streak of 4 SCs, Les Habs won 4 straight SCs in the '70s.