I would have initially voted for The Racer, for most of the reasons listed in this thread, but this point about The Beast is what makes it the winner. for me The Beast made KI a destination, in the sense that people from around the country (world?) who otherwise wouldn't have been to KI, had to get there to check out The Beast. I'm not the historian many others on here are, especially when it comes to other parks. But how many other parks at the time had a ride as grandiose and magnificent as The Beast? What other parks/attractions would people have said "I have to visit this park just so I can experience this ride?" I'm sure there were others, but not many. KI dreamed big, took a huge swing, and built something most people wouldn't have ever conceived in their own imaginations. Doing it almost completely in-house just makes it even more impressive. I think it ultimately influenced how other parks thought about coasters. It changed the perception of what a coaster could be, in a way that led to bigger, faster, and more epic thrill rides.
I'm biased, because I grew up as a kid in the 80s and early 90s, and The Beast always just had this aura. You could see the lift hills from various points in the park, and the paw prints leading you there meant The Beast was always in the back of your mind, whenever you were in the park. I still get chills and butterflies when I approach the queue to ride The Beast. So maybe those feelings warp my view of it. But I still think objectively, you can't deny the impact it had, simply due to its magnitude and uniqueness.