I've read pretty much everything Stephen King has ever published, except the later books of the Dark Tower series as I just couldn't get into that.
My favorite, without doubt, is It. It's an opinion shared by a lot of fans and also by Stephen King himself, but It is 1,100 pages long and certainly not the quickest or simplest introduction to King's work.
My second favorite is From A Buick 8. Extremely unique idea which is beyond creepy and disturbing, especially when you realize it was King's WIP when he was hit and nearly killed by that car in real life. It's also average length and has no connection to his many other works set in Maine, so a great stand-alone introduction to his writing and style.
If you want to start with "classic King", I consider 'Salem's Lot the best of his early/most famous novels. It is just a classic vampire story, but extremely well-written and well-paced.
I also can't recommend his short story collections highly enough. He's a master of the shorter forms (short stories, novellas, and novelettes) and you truly can't go wrong with any of his collections. My personal favorite, though, is Full Dark, No Stars, which is a collection of four novellas that stand together as one of the most disturbing books I've ever read.
In the end I doubt you'll get much of a consensus from King fans about where to start, though. Many people think The Stand is his best work, and I think it's his absolute worst, one of his only books I couldn't bring myself to finish because I was bored to tears and there were STILL 400+ pages to go. On the other hand, I adore his novel Needful Things which many others call a silly idea with a hot mess of bad plotting (I feel like they're missing the points, lol). He's written such a huge variety of stuff that the best recommendation is probably for you to check out the free Amazon samples of the various books I and others recommend and choose the one that pulls you in most and sounds most personally interesting.