I don't care what people think. Left 4 Dead 2 is still scary as L4D1. The dark mall where the only light source is the sun light from the skylight, with the ambient noises, so good. Or the streets with no people other than zombies, with the sun setting on the horizon.
L4d2's soundtrack is scarier imo. The banjos and violins are extra stringy, the trumpets are extra brassy, and the notes themselves are ugly yet beautiful when put together. L4D1's soundtrack definitely sets a creepy tone during quiet moments, and the music queues for special infected are unsettling but during combat it's just drums and synths. I'd say the organs blaring throughout Dark Carnival not only enhance the tone of that chapter, but it also makes the combat sequences scarier. That applies to every variation of "zombat" music within L4D2.
The suburbs of hard rain is also pretty creepy with the light rain and sounds of the storm in the background. While the houses all dark inside.