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

I've been researching and looking more into room treatment products. Wondering what product really worked for you?
21192e21 5d00 4ded bea1 72869e5cf35dscar972
@mijostyn Don’t use those acoustic foam tiles. They sound bad. I had a bunch of them and thought they took the edge off but something wasn’t right. When I removed them, everything sounded nice again. They distort the sound and add a grey harshness.
REW software, a UMIK microphone, and time to learn about acoustics and measuring.

Rooms are so different, as are preferences, that until you really know what you want to change and why, suggestions about products just creates a needle-in-a-haystack problem for you.

Once you spend some time learning (including critical listening) what you can do with what you have (speaker position, listening position, objects already in your space), you can then move on to what needs to be adjusted -- and which product types will do that.

Critical drywall angles on ceiling.
Could you elaborate?  This sounds similar to something I am doing.

Hilde45, You are a wise man!  :>)

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?
I made (8) 4' x 2' x 2" panels, with fabric, mounting hardware and shipping costs for just over $200 with products from ATS Acoustics.
I mounted 6 of these in my home office and it made a dramatic improvement in the sound of my system.  The other 2 panels got mounted in our family room to help tame some of the echo that was present.