R8V9F5A2: To me 'must see' inevitably leads to recommending what I consider to be classics.
I'd first have to say Peter Jackson's
Braindead and
Bad Taste.
Although these are probably more comedy than horror in a way.
The Thing, The Fly, Omen, Poltergeist, Children of the Corn, Day of the Dead, The Hitcher,
Invasion of the Body Snatchers, Evil Dead, Amityville, Leviathan, Pet Cemetery, Ring, Ju-on.
I always thought 'Day of the Dead' was the weaker of the series. Mind you I really love them all, I just thought 'Night' and 'Dawn' were better. Honestly I could barely sit through 'Land of the Dead' but I thought Romero bounced back VERY nicely with 'Diary of the Dead'.
Wait....I just realized most of the names you posted have been remade, some for better, some for much worse, so I guess you should clarify which is which. :D