1. Mulan- Shan-Yu's forces (Huns) colliding down the mountain pass
As a kid, this scene blew my tiny mind. The silence in the beginning is so haunting and the sense of doom just lingers until the smoke clears and you see Shan-Yu at the top of the mountain. When the drums start beating and the Shan-Yu's men reveal themselves as a force of thousands, it automatically becomes painfully real for the characters and the audience. This scene scared me the most when I was a kid, because watching the bad guys completely engulf Mulan and Shang's army was truly intense.
Fun Fact: 1,000 Huns were drawn specifically for this scene
2. Tarzan- Clayton's death
Clayton's death takes the cake for being one of Disney's most darkest moments. The vines contrast the spurts of lightning while shadowing Clayton and Tarzan. The foil between dark and light intensifies the battle. This scene in particular reminds me a lot of the fiery showdown between Simba and Scar at the end of Lion King.