Thunderstorms
Sunlight
Tornadoes
Hurricanes
Aurora Borealis
Plant growth

The coolest thing I ever saw personally was the occurrence of a water spout out in the Mississippi Sound one summer evening. Like a water tornado happening over the ocean.

Really, when I think about it, gravity and the fact that the earth rotates and day changes to night and back again are just amazing in themselves.

Ooops! I nearly forgot one! Childibirth. That's an amazing miracle, too. Just incredible.