What are the best Room Accustic Treaments members have found?

I  am looking into adding some room acoustic treatments to my room.  I am just looking for advice on some simple room treatments that fellow members found worthy of purchase. 
ieales516 posts12-05-2019 2:04am" Books, shelves, art & carpets.

But then, I spent a few decades in sound studios, so I'm probably odd man out here. "

This must be basic. No audiophile should ever put up big pictures with reflecting glass-fronts. We need to be aware of the impact a leather sofa has in the room versus a velour sofa or chair. Know that real paintings made on canvas are wery efficient diffractors and will eat flutter-echo. Decorative wooden blinds are actually adjustable diffractors and will calm down big windows or mybe a wall. Avoid naked corners, try to brake them up with something. I have a big plastic tree in one of mine, next to a classic painting with a matching motive. Females looks at this and give me an appreciative look. If they only knew..

One thing I can say without fear of contradiction that does not (rpt not) work is SONEX foam panels. You know what I’m talking about, those sculpted dark gray open cell foam 2x2 panels that are ubiquitous in control rooms and recording studios. SONEX foam and similar products, even foam filled chairs like the IKEA POANG chair, have to be one of the worst sounding materials ever foisted on naive and gullible audiophiles. Even a little bit in the room makes the sound unnatural, phasey and wooly.
+1 for Primacoustic panels. Purchased mine from Sweetwater Sound. Placed them on all four walls plus the ceiling at all first reflection points. Enormous improvement in SQ. More than any gear change. Remember, the room and the setup determines at least half of what you'll hear. 
Also +1 for Jim Smith's book Get Better Sound.
+1 for avoiding a foam solution. It does not work evenly over all frequencies and is also a fire hazard. The best engineered panels are made of 6lb per cubic foot fiberglass. The way they work is that the acoustic energy hits the fiberglass causing the fibers to vibrate. The fibers vibrating against the air causes friction with the air turning the acoustic energy into heat... Something like a millionth of a degree or less... 

Fiberglass panels typically keep their shape with an acoustically transparent covering and a resin frame. 

Acoustic diffusers work great for diffusing the sound but do little to absorb excessive acoustic energy in a room. Furniture, rugs and carpeting can help absorb excessive acoustic energy in a room. But the problem is how much and what frequencies. Well engineered acoustic panels can help control a room in equal increments. Have a softDr room? Try 15% treatment. Have a hard room? Try more treatment but keep it to a max of 30% or risk making the room dead or dark... Unless, of course, dead or dark is your purpose or pleasure. Forensic labs might use a 50% treatment of 2" panels.

Because rooms vary so much in construction it is difficult to predict acoustic behavior by guessing.
There is no need to guess, for less than the price of a pair of modest interconnects or isolation product which may or may not have an effect, a suitable microphone and free download will allow you to determine the problem frequencies and then treat them with absorption. This will have a profound effect which will elevate your system's performance to the point where perhaps a component upgrade that was being considered is now no longer necessary. It is, if done correctly,

The suggestion of crowded bookshelves, curtains, carpets and clutter is no more than guesswork. Adding any or all of these will have an audible effect, unfortunately it probably isn't an improvement. The books will just collect dust and the drapes and carpet are only narrow- band absorbers.

@gaffekait You scoff at my mention of E. Winer. Why is that? He is certainly more on track than Acoustic Fields. Forget the guy's name, Foley or Folly.  I could have mentioned the famous acoustician Leo L Beranek but it is very technical and does not make for easy reading. As the OP is clearly not knowledgeable on acoustics, steering him towards Beranek probably would not help. The OP may have no interest in any reading anything so I felt an easy intro into the subject could be to watch a short video.