For me it was Medal of Honor when I was in the landing craft just having spawned there at the beginning of the game. I was freaking when the first WHOOM of artillery hit the water next to us. Then, I saw a landing craft get hit and the bodies flew in the air. I thought WOW this won't be toped. I was wrong. When the gate dropped and I hit the beach a hail of gun fire rained down like hell on earth. The bullets were whizzing by and I thought there was no way I could survive this. But I did and made it to the low sea wall.
This MOH moment was the most immersive moment i had ever experienced. This topped even my love of boss battles in Quake.
Here is a video of exactly what I remembered:
The above link is a great video of the D-Day portion of the game.
Why so many edits? I was learning how to make links.