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

Free standing GIK 244 panels for first reflection points.  The "free standing" version. allows you to move them as desired to tailor the sound. 

Critical drywall  angles on ceiling.
Lambs wool panels at 5 locations woven wool carpet 2 home grown music directors on floor 1 music fan oh and 3 Argent Room Lenses silly for some dilberts inspiring for those with ears and mind. Tom

@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.

@theaudiotweak 

Critical drywall angles on ceiling.

Could you elaborate?  This sounds similar to something I am doing.

Hilde45, You are a wise man!  :>)