I'm gonna go against the grain and say maybe don't start with King at all if you're a seasoned horror fan. He's foundational, but his voice is very specific. Try something by Clive Barker or Shirley Jackson first, then come to King to see where a lot of modern horror tropes got polished. Sometimes context helps you appreciate him more.
For a pure, unadulterated blast of classic King horror, it's hard to beat 'Christine'. Sentient, evil car? Check. 1950s nostalgia as a corrupting force? Check. A slow takeover of the protagonist's best friend? Check. It's straightforward, viciously fun, and doesn't get bogged down. It's also a great look at his themes of addiction and obsession, wrapped in a greasy, motor-oil-scented package. If you want a pulpy, addictive page-turner that embodies his 80s era, start your engines here.
Ultimately, there is no single right answer. His career is too long, his range too wide. A recommendation is just handing someone a key to a specific door in a vast, creepy, wonderful mansion. The best you can do is listen to what they like in other stories and point them to the door that seems to fit their key. They might just wander through the whole place after that.
Carrie' is a solid, concise starting point. It’s his first novel, so it’s where his voice began, and it’s mercifully short. The epistolary style with news reports and articles makes it feel immediate and real.
You get the classic King themes of bullying, cruelty, and explosive revenge, all wrapped up in a tight narrative. It’s a quick read that delivers a huge emotional punch, showing his knack for making you sympathize with someone who does terrible things.