What are the best Room Accustic Treaments members have found?

David, It all depends on your room and the type of speakers you have.
For primary reflections over 200 Hz acoustic tiles work great and are dirt cheap.  https://www.parts-express.com/sonic-barrier-fwp122-studio-acoustic-foam-wedge-panel-12-x-12-x-2-black-12-pack--260-547
You can find your primary reflection points using the mirror method then place these tiles at those points. Your image will solidify.
Dealing with bass is far more complicated. The best way is to use multiple subwoofers. 

It depends what you want...
I worked with vicoustic and submitted a project to them, they came back with 3d renderings and room modes analysis and treatment suggestions.
I believe other companies offer similar services.
I used diffusers and absobers, mainly on reflective points and it brought a sense of ease to the music. I didn't go all the way because I didn't want to turn my living room into a mastering studio.
If you want something even more aesthetic, I would look at artnovion.
They have some good looking products.
Others mentionned GIK, it's also good.

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.