The Dresden Files, which I've found don't necessarily have to be read in order, although that would be best, are some very entertaining urban fantasy/action books that have some very unique, awesome scenarios in the them. I suggest these because of your mention of Koontz and King, and because they are most excellent.
On a completely different note, I'd recommend "The Book Thief", which is about a non-Jewish girl growing up in Nazi Germany. I don't usually like historical fiction overmuch, but this book is extremely touching and though provoking. One of my favorites.
If you'd like to try some science fiction, "Dune" is one of the best soft scifi books there is, and "Revelation Space" by Alaistar Reynolds one of the best hard scifi. Both of these books, although very different from one another, feature very complete, well-realized worlds, and are among my favorites.
For fantasy, I'd recommend "The Name of the Wind". It's a very, very thick book, is extremely well written, and features an incredibly detailed and well-realized world, as well as likeable characters. The main character is somewhat over the top, but the whole book is done so well I'd have it no other way.
For humor, try "The Hitchhiker's Guide to the Galaxy" by Douglas Adams, or anything else he wrote. His books are like nothing else, and are utterly hilarious. Anything by Terry Pratchett is also excellent. Some of my favorites are The Bromeliad Trilogy and Going Postal. His books are extremely funny, but he also squeezes a good deal of meaning into them. His characters, including his female ones, are very real, and while he pokes fun at humanity, it is also abundantly clear how much he likes us.
I admit, some of these books (the bromeliad trilogy, the dresden files, the book thief, the hitchhiker's guide) are occasionally shelved in the young adult section, and the Bromeliad Trilogy was specifically marketed for young adults. However, that is NOT why I recommend them. I recommend them because they are simply awesome books. They are NOT the sort of young adult aimed fiction that is full of touchy-feely boy meet girl stories, so called life lessons, high school, and "teen issues". I would recommend them to anybody, young or old, who has a taste for good books. Anyway, only the Bromeliad trilogy was marketed specifically at young adults (but it's not written down, or really all that different than his other books. I think it was just a marketing ploy.) The others seem to get thrown into the YA section sort of at random, in an attempt to introduce some variety to the section that is typically packed full of boring teen books.
Anyway, I hope this babbling post is of some use to you.