So many things, I hardly know where to begin.
The green flash at sunset (some people think this is an illusion, but it's real).
I walked across an active volcano (on the Big Island of Hawaii) and looked down into the fire pit one day before it blew up and covered the road we had driven to get there with lava several feet deep.
My wife's face when I came out of anaesthesia after surgery.
Most overwhelming thing I've seen: a tornado one street over from our house, popping roofs off of neighbours' houses. That close, you don't see the cliche funnel shape -- the whole sky goes dark when it's that close. The roaring noise is deafening, the sky is full of swirling debris. It looks like divine retribution.