I have two.
Aliens is the first. Great story, excellent performances (Weaver is such a badass in this one), and overall an incredible movie. The special effects look good even today, and it's one of if not the most exciting movies ever made.
The other of course would be The Empire Strikes Back. While I love the other two films of the original trilogy almost as much and greatly acknowledge their impact on the movie world and feel they're worthy of the praise they get, at the same time I feel they're little more than just fun sci-fi thrillers. Empire stands out to me as a truly terrific movie. The characters are actually given an incredible amount of depth and development (moreso than in the other films), and overall it has a much darker and serious tone that greatly elevates beyond the realm of the normal fun sci-fi thriller (which is what I felt the other two essentially were, even if they're worthy of their merits). And the ending is still incredibly shocking, even if it's beaten into your head a billion times.