I think The Beast (after re-tracking) is still one of our most popular rides that people travel to the park to ride. Some people may come to ride Mystic Timbers or Orion but those are probably the biggest rides with drawing power. Nobody plans a trip to Kings Island from out of town to ride Dback, Flight of Fear, Banshee, The Racer, or The Bat which are all top tier rides at the park as well (at least in my experience with out of towners I've talked to.
As for the Orion VS Dback debate I personally think Orion is a good ride and it's far superior to Firehawk (which is what it more or less replaced). However, Orion has a few elements that don't do anything for me including the wave turn (top of second hill) and the "belt" section (helix). The ride also just feels short and while any row delivers good airtime, the back rows of Diamondback are better than any row on Orion. Diamondback is longer in ride time and provided you are in the back always delivers great airtime over every hill. Diamondback is my personal favorite coaster out of approx 170 credits.
Now I personally LOVE millennium force and Magnum XL 200. They are my #2 and #3 right behind Dback. These two coasters are a perfect example of what makes a good hyper/giga combo. Magnum is an airtime machine and milli is all about the positive Gs and speed. Two vastly different rides unlike Orion and Dback which are too similar in my opinion. The contrast of Milli and Magnum at KI would be Dback and Beast to me. Dback has the airtime like Magnum and beast is a coaster built for speed much like milli. Did I forget to mention beast is my #4 of all time? (Sure I'm a little biased towards Ohio I think)