What room treatment product have transformed your room or audio system?

I've experimented with a few different products and the best result so far is a combination of panels, diffusers, bass trap and a lot of time listening and adjusting.
Like @noromance I don't recommend acoustic foam, they have a deadening quality and is more suitable for recording studios. In a listening environment, the frequencies needs to be controlled but remains lively sounding. The first reflection on the side wall next to the speakers are the most critical and makes the biggest difference.
The best I've tried so far is a discontinued diffuser product by Auralex similar to GIK Gridfusor, these diffusers just sound so natural and balanced but they are not the best looking. I'm wondering if anyone has experience with GIK PolyFusor or Auralex T'Fusor on the side walls?

Some interesting products mentioned! I haven’t thought of digital room control before.
I’ve been listening and adjusting the past few days and have gotten to the point where I’m really satisfied, a combination of panels, bass traps, diffusers, and hybrid products.