Though it might be impossible to choose one thing that's "best" about Carrie, I think the juxtaposition of its two shower scenes is definitely in the running. I've never read the Stephen King book so I don't know which of the film's strengths to attribute to the famed horror novelist but Brian De Palma unquestionably did a lot of things right in the transfer.
The first triumph is his undiluted understanding of Carrie's sexual development as adolescent terror themes. The second is his facility with cinematic language. In the opening shower sequence there's slow motion bodies and soft music. Carrie herself (Sissy Spacek) is completely entranced by the water, lost in the pleasure of it. Until her hand, soaping between her legs, comes up red with blood. Carrie raises her trembling hands up and out, staring at them in a confusion and disbelief that shifts quickly into pure terror. Her screams for help invite mocking from her classmates. Carrie crumbles into the corner until her gym teacher rescues her.
The second shower sequence, even more famous, is its twisted malevolent twin. Carrie isn't any where near a literal shower this time. Declared prom queen, she walks to the gymnasium stage in slow motion, that now familiar sad and soothing theme playing again. Carrie's eyes are entranced and wet, lost in the pleasure of acceptance and applause. Until, standing on the stage, her sadistic classmate drops a bucket of pigs blood on her.
Two showers of blood, then, the second infinitely more garish with the stuff. For the second time Carrie raises her bloodied hands up and out, staring at them in a confusion and disbelief. This time, however, her open mouthed shock produces no screaming for help (none that we hear at any rate in the expressive sound design) and her emotions shift to fury rather than terror.
No crumbling in the corner. No waiting for rescue.
Psychotic break time. (God bless the split screen!) This time it won't be Carrie doing the screaming.
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