At first I thought it was a tie between 6 and 7, but now it's definitely 7 for me.

Spoilers ahead:
- Midgar itself is dark and gloomy
- Jenova's escape from the lab. God that was scary. All she left was a long trail of blood.
- Midgar Zolom carcass
- Sephiroth clones
- Aeris's death
- The world after Meteor is summoned. Even the music is dark and gloomy.