Um sorry but, Millennium Force has no airtime and shouldn't even be in this poll! Even though I love it to bits, it still has no airtime.
Out of that list I would say it's a draw between Magnum XL-200 and Superman Ride of Steel.
Magnum is often overlooked due to being so old, and most people have gotten over this once new and innovative concept long ago, so it doesn't really ever get the credit it truly deserves. I still love it though and it's still extremely insane and intense, especially in the front.
S:RoS looks like it would be a good coaster as a whole more so than Magnum, since it has a mix of everything. Airtime, a bit of a twisted layout, and bits that go underground. But, since this poll is about airtime, I'd say the airtime looks good. I'd assume it's one of those things you have to ride to really understand, so I can't properly judge it yet but yah, it looks good.
So Magnum and S:RoS.