I think most King readers would vouch for The Stand as one of his best. That is a great book to start with (although it is really long), because it typifies his writing style and is a fantastic read. Even people who don't like King generally enjoy this book, and it is definitely among my faves as well.
Personally, The Talisman is my favorite (although he co-wrote that one with Peter Straub), followed by The Dark Tower series, and The Girl Who Loved Tom Gordon. His short story collections are also really good, with Four Past Midnight and Different Seasons at the top of that list.
Personally, I generally don't enjoy Koontz's books. From the Corner of His Eye was good, as was The Watchers, but I think his books are much more formulaic than King's. Also, they both write about very weird topics or events, but because King puts so much effort into character development and focuses on how "the weirdness" affects the characters vs. "the weirdness" itself, the stories become much more personal and believable, IMO. I feel like I'm not explaining this clearly so I hope it makes sense, but the bottom line for me is this: Koontz has written a few books I like, but King has only written a few books I don't like.
That's just my opinion though!