Best products for baffle material s ?

The best material would have extreme tensile strength and Young's modulus of elasticity, but also high internal damping. Metals can of course have high strength, but very poor damping. And if you search, one can find tables which list the Rockwell Hardness for many different species of hardwood, some can be quite high. And wood of course has high damping. Metals do have the advantage of mass, which means more energy needed to excite resonances, but lack of damping means energy storage or 'ringing'.

Wood in it's many forms is a good compromise. MDF however is typically too low in terms of strength. But there are some high-density ultra-refined MDF variants which are better.

Of the modern materials available today, thermoplastics such as phenolics I would have to say are best. They come in 1000 forms, unfortunately most are very expensive. Spec sheets are also available which list the parameters mentioned above. But some can combine tensile strength higher than 6061 aluminum, AND excellent internal damping.

PTSS, I haven't read every post here, but I think your
premise, that a less resonant cabinet is better, is simplistic
and flawed. Every material, regardless of density and weight,
has a resonant frequency. Companies like Spendor and Harbeth,
who both produce excellent products, have opted to go with a
thinner, more resonant box in order to shift the resonant
frequencies into the range they wished them to be. This takes
time, sophisticated measuring equipment and (most of all) a
clear idea of what the maker wants the finished products to
be. Your approach of just building a non-resonant cabinet is
nothing but trial and error and the misguided theory that
heavy cabinets are automatically better. Just to be clear,
there are many companies using heavy cabinets with great
success, but again, the process is not haphazard. At least it
shouldn't be. My advice - if you don't like what you have,
sell them and buy something you like. Unless off course, you
just want to screw around with a project, which if fine. It's
your stuff and you can do whatever you want with it. Good
luck.

Chayro - you have things completely backward. It's building resonant, colored cabinets that is completely trial-and-error, and based on misguided theory. Strong, massive, resonance free cabinets ARE better.

The job is to create an instrument as true to the source as possible. And cabinet resonances, intentional or not, are pure 100% distortion that are delayed in time and uncontrollable in relation to volume.

If the designer intends to create a colored/innacurate music reproducer, there are ways one can do that in a controlled and predictable way. Cabinet resonances are not. The reality is that some companies choose to make resonant boxes not for performance reasons but budget reason, they are always much cheaper and much simpler to build. They then tell the marketing dept to come up with ways to obfuscate the truth.

Vapor1 - You misinterpreted my post. I am saying that the OP
believes that if he builds a heavy enclosure, it will
automatically be superior to the one he has now, which he
believes vibrates. He may well end up preferring what he has
now. I am saying that cabinet design is much more complex
that gluing together some heavy pieces of material. And
Vapor1, if you're saying that Harbeth and Spendor speakers are
built on misguided theories, well - I guess you have the right
to your opinion.

I'm not sure using identical height and width would be ideal. In some designers more ambitious designs they use triangular baffles, or at the very least sides with different dimensions.