For me it would probably have to be April Fool's Day (1986).
I originally saw it back in the theater when I was just 11 years old, and the ending completely shocked me back then. Before that, I'd probably never witnessed such an ending shocker.
"Through unlit corridors of purest oblivion.
Submerged in a sea of darkness without end.
To feel nothing...hear nothing...for a thousand eternities." - Evoken - Quietus