1. MIB's speech to Jacob on the beach.
“They come, they fight, they destroy, they corrupt. It always ends the same.”
“It only ends once. Anything that happens before that is just progress.”
2. Bearded Jack telling Kate that "they had to go back," which was actually a flash forward scene.
3. The Hatch during a lock down, when the count down timer goes hieroglyphic "He Escapes Place of Death," and Locke finds the Blast Door Map.
4. Flight 815 tearing a part in mid-air.
5. After Ben saves Locke from committing suicide, Ben strangles him to death.
6. Ben "summoning" the smoke monster by pouring water down a old drain after Keamy executed Alex.
7. Eko speaking to the smoke monster in his brother's form. Eko refuses to relent from his past, which angers the smoke monster into killing him. Eko whispers his last breath to Locke, "you're next."
8. The Others attacking the raft and taking Walt.
9. From the viewpoint of the rescue helicopter, the island vanishing after the frozen donkey wheel is turned by Ben.
10. Desmond turning the fail safe key which causes and explosion, implosion and purple flash of the Hatch being destroyed.
This is not an exhaustive list. Some shocking honorable mentions include:
Michael murdering Ana Lucia and Libby.
How Locke was thrown through a window by his father.
Dying Charlie note written on his hand to Desmond, "Not Penny's Boat."
Desmond's phone call to Penny in "The Constant."
The O6 return to find Claire a crazy jungle lady with a mummified squirrel baby.
The shining moment to LOST were these type of memorable scenes. They continue to linger in our collective consciousness. Those moments got us thinking and talking. Those moments kept us coming back for more. Those moments kept us driving for answers to our personal questions. Those moments kept us from becoming lost in our ordinary lives.