In the category of Classics I've Always Been Ashamed To Admit I've Never Seen (but now I have, so I can admit it):
I finally caught up with Strangers on a Train. It's all that everyone says, and then some. Everyone talks about Robert Walker's performance, rightly, but even so, it surprised me. First, I thought I had seen him before but it turns out I hadn't (his other movies don't come up often). From descriptions of his role I had expected something obviously creepy, a Peter Lorre sort of persona, but this is much more interesting: someone who's physically a rugged All-American kind of guy, who is also insinuating, vaguely gay-flirty, and yes, creepy, but in an entirely original way I don't expect to see in something from that era. And then there's Farley Granger's soft passivity (perfectly used here): it's needed for the plot to work, so the character doesn't just brush the intrusive stranger off in the first minute and have nothing more to do with him, but it also interacts fascinatingly with what Walker is doing.
And of course there are the often-mentioned Hitchcock setpieces: the informal tennis match with one motionless set of eyes, the climactic match intercut with retrieving the lighter, the runaway carousel.
Some "classics" don't live up to their reputation. This one does.