Okay, now for a couple of serious ones:
The launch on Formula Rossa
The acceleration. It goes on and on. And on. You're kicked into a roaring stream of air, but the kick keeps going until you're going faster than what seems sensible. And then up a hill, and you're high in the air before you can collect your thoughts. The trim brakes aren't really that noticeable either, you're still going insanely fast.
Thundercoaster's first hill/second drop
Had to mention one from my home park (well, technically not anymore?). The lift hill on Thundercoaster goes along a hillside, so it doesn't feel like you're that high up when the ride begins, especially if you sit on the left side. Then a sweeping drop begins, you turn, dive under the structure of the lift hill, up a crest...
...and suddenly you're in mid-air, or so it feels. The only structure taller than you in the immediate area is behind you, so everywhere you look is down. As I said, the lift hill goes along the side of a hill, and the first drop turns towards the hillside, but now you've done a complete 270 degree turn, so straight ahead you're looking down the hillside, further enhancing the feeling of being high up. And this first hill is also the site of Thundercoaster's steepest drop, and is (well, was) taken at quite some speed, so you're thrown out of the seat, or so it feels like. You've crested the hill and your body has the momentum to continue a forward motion, but the train and its lapbar pull you down instead. So you're hanging there, in mid-air, while the coaster drags you in a direction that feels like straight down. And then you hit the tunnel. A shame the ride is a total roughness-fest from there on. It was quite amazing in its first few years, though.
Also seconding all the Helix statements. It's a great coaster.