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

[design3]

I use Ableton Live; maybe try out the trial (30 days, allows exporting) because you could honestly make a ton of ambient/drone tracks in that period of time. Even just time-stretched noises with heavy reverb and bass EQ work great for low-level ambiance.

In short, you can drag in a sound, slow it way down, add reverb, throw some compression over that, and you'd be on your way to ominous background noise.