In no particular order (but only becuase I'm mentally going back through my own DVD/VHS collection and over memories of films I've seen and admired over the years).
"Forbidden Planet" (Walter Pidgeon)--A classic. 'Nuff said (don't forget the formative beginnings of the genre! And frightening too it it's own way)
"2010" (Roy Scheider)--Fantastic film and very frightening (and uplifting too) on so many levels.
"Alien" (Sigourney Weaver)-- the second film, "Aliens" was fantastic too, but it would not have mattered as much, but for the original. The first one, in and of itself, truly gave new meaning...and fuel...to the simple phrase "night terrors."
"Star Trek III" (Leonard Nimoy)--"ST II" was fantastic, but, IMO, "ST III" still trancended it--"ST II was "about the death of a resiliant and tough enemy," while "ST III" was about the death of an honorable and well loved friend (and so the latter trumps the former. Or (and to borrow a well-turned phrase)...
.... "the needs of the many (the latter) outweigh the needs of the few (the former)...").
Last: "Total Recall" (because is was mind-expanding in so many thoughtful ways (if not so much now, this was at least true at the time)).
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Honorable mention: HEHEHE...I'm mostly kidding, here, so DO take it with a very large grain of salt, please: "The Reluctant Astronaut" (Don Knotts).--Best scene ever! involving only "Peanut-Butter-and-Crackers" (my seven year old nephew really loves it to this very day. As did I too, when I liived, myself "back in the day.").
Hey...It's not always about being "frightening--sometimes it can be aboutsimple (and even marginally dumb) humor too!
).
CS