I have to pick just one???
aye-yi-yi
let's see, let's see..... nope,
Ghostbusters was '84 so I wasn't quite a teen yet.... um.... anything I saw in college is out (rules out everything from
Heat to
MIB) ....
wow, this is hard. There are a lot of films I could list.
Field of Dreams,
Full Metal Jacket,
Die Hard,
Silence of the Lambs, even
Dances With Wolves) - as yes, I am something of a cinephile, and I suppose I ought to restrict it to films I actually saw at the time rather than those that came out then, which rules out films like
Miller's Crossing....
but, if I had to pick one, with the criteria that it would be the only film from the era I could own, for example, it would be
The Princess Bride. To pick anything else would be inconceivable.
(though
Unforgiven merits a shout-out, as being a favorite and one of the last films I saw as a teen

)