Hmm, I have four moments that rank equally on my "most shaken" list:#
1) When scratching my neck, I felt a hard knot next to my spine. Thank god, it was just an infection of a lymph node and not cancer.
2) When a doe got partly into my car, through the windscreen at 80 kph. It was the only time in my life I had kind of a panic black out. Not that I passed out, but there's a gap in my memory from going along the road nicely when my then GF yelled "DOE!" to standing on a bycicle lane next to the road, with the car and myself full of doe fur and broken windscreen glass, and a thankfully quite dead but not brutally maimed doe next to the car.
3) Driving home on the busy Autobahn, going 140-150 kph when right out of nowhere a bike lay across my lane. I had no room to swerve left or right due to the heavy traffic and me and the bike being in the middle lane. Everything slowed down for me, including my perception of my own speed of reacting. Still I remained strangely calm, seeing the bike come nearer and nearer and realizing that when I hit it, I will most likely spin into another car at a very high speed, causing quite a severe accident.
Surprisingly, I squeezed past it.
I immediately called the authorities, and while phoning, I saw a hungarian car with a bike stand on the roof parked on the emergency lane. I described the car to the police. Sadly, the police wasn't able to clear or at least close the lane quickly enough, so the bike caused a heavy accident with severe injuries (but no fatalaties). Due to my help though, they got the culprits before they were 20 kilometers away from the scene.
4) Going on a well known road in the middle of a foggy winter night. The speed limit is 80, but most drivers drive 100 as it's safely possible to do so and no speed cameras around. That night though, I felt uneasy and I slowed down to 60. Then, at a right hand turn leading into a downward slope, I hit black ice. The steering got light, I got on the opposite lane, spun out to the right and skidded down the whole slope blocking both lanes. The 200 metres I slid were the longest in my life, with nothing I could do except bracing for impact in case I hit the guardrail, miss it and drop down the embankment, or an oncoming car which would be unable to brake due to the very ice I slid on.
Thank god none of that happened, and I could continue after having a few relieved deep breaths. Turned out that from the point I skid, the road was a massive surface of ice all the way home, which were another 10 kilometers.