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

Stillpoints Aperature Panels series 2 8 were transformative in my room.I have tried Tube Traps,Echo busters etc these are by far the best.Not cheap but worth every penny. 

Vicoustic DC2 Skyline diffuser combined with GIK corner traps and absorption panels and level 3 HFTs provided an excellent and balanced sound. 

Been using ASC half and quarter rounds for years now along with their panels and have had great success in every room they’ve been used in but the new ASC Isotherm traps are a big leap forward in performance. Increased absorption factor, deeper reach and improved articulation. 

Are you handy?  I used Owens Corning 703 panels for first reflection points on sidewalls and ceiling.  Wrapped them in fabric, very easy and inexpensive.  Made a huge improvement in soundstage and imaging.  Just be cautious while handling (mask, gloves, long sleeve shirt, etc.) and do out in the garage if you can.  

Whatever you decide, just start with first reflection points and then see how you like it.  It might be all you need.