What's your best program?The program use for what?

To specify: the best program I've written or the best program/piece of software that I use on the regular?

The best thing I ever wrote from scratch was the software for my magic mirror project a while ago, using a Pi and a bunch of Arduinos to handle gestures with a PIR sensor.

The best piece of software that I use regularly is probably f.lux (or Redshift on Linux) because it has legitimate influence on my actual IRL life, helping me sleep and just making my nights more physically enjoyable.