Feel free to give several examples if you can't narrow it down to one.

This way we can all get great play tips for really specific experiences.
I'll go first as an example.
Mine is probably the first level of Ikaruga because it set the theme and style of the entire rest of the game.
Seeing the last level be so cold and isolated felt like the perfect polar opposite to the first world where you learned all the basics and final level put those skills to use.
Oh and hi everyone I'm new to this forum=D *wave*