What's the best program to use to make ambience?

[Curunir]

FL Studio is neat because it's perhaps easiest to pick up.

As for ambience, you could either become a sound wiz and engineer your creepy sounds or use some good software synth. What would work especially well for creepy ambience I imagine is stuff like Native Instruments' Absynth, Krakli's Cygnus (freeware too), Green Oak's Crystal. All those come in the standardized VST format which you can use in a number of host applications.