Who's your favorite actor? De Niro? Pacino? Brando, maybe? I like those guys too, but the fact remains that no film actor ever surpassed Lon Chaney in terms of technical ability. Chaney can do more with just his face than literally any other actor can with many more tools at his or her disposal: Dialog being the big one of the bunch.
I definitely do NOT want to spoil the plot of The Unknown, because part of its berserk pleasure is watching its unpredictable plot unfold. But as someone who has seen this movie around half a dozen times, I can vouch for the fact that it holds up even after you know everything that happens in it.
I'll give you the briefest of setups here, just to get you interested: Lon plays an armless circus performer whose specialty is doing stuff with his feet - throwing knives, firing guns, that sort of thing. Joan Crawford plays another member of the circus troupe with a bit of a psychological hang-up: She can't stand being pawed at by men. A match made in heaven? You'll have to watch the movie to find out, but suffice it to say that their would-be romance does not progress smoothly.
In lieu of an in-depth and spoilery discussion of The Unknown's plot, I'll relay an anecdote of my first viewing of it that I think illustrates its power even 80+ years after its release. I was visiting home from college, and put on The Unknown as part of a recently purchased "Lon Chaney Collection" DVD set. My sister, 7 or 8 at the time, happened into the room and started watching it too, her interest becoming stronger as the movie progressed. By the time the final scene arrived, she was in near hysterics over what was going to happen. Never let anybody tell you that silent movies "don't hold up" while movies like The Unknown exist.
Sorry about the vague nature of this post - I'm just really reluctant to spoil this movie for anyone because I recommend it so highly. It's probably my favorite of the movies written up here so far, which, looking back, I guess isn't that impressive. But it's also one of my favorite horror movies ever, and I can't recommend it highly enough.