Well, I still recommend the creepy 1965 The Collector
Black Christmas tops my list--at least of slasher films--too. Plus it's Canadian (of course a lot of those 70s/80s slashers were filmed here, but had less actual Canadian actors/crew). It's truly brilliant, has (for me) just the right amount of gore--plus I feel good watching it when I'm feeling grumpy at Christmas time. (And seriously what a cast--Olivia, Keir, Andrea, Margot...)
I think, despitesome truly dark subject matter and that creepy ending, it also has the right balance for me of not seeming misogynistic--as so many later slashers did--and having characters you actually find interesting and some you care about.
I am sure any fan knows the story, how John Carpenter was loosely involved in it, and probably heard that at one point Bob Clark (who went on to do the Canadian "classic" Porky's as well as A Christmas Story) was contemplating a sequel set at Halloween... Not to take away from Halloween's brilliance, though.
I know it had a bit of a bothced release--different titles in different places, no major distribution, but I'm glad it's become the cult classic it has (I obviously ignored the remake though I heard it involved eating eyeballs...)