Which material sounds better for speakers construction? Wood, Ply or MDF?

How can you ask that question 

You'll all full of schitt !!! Dr. Bose got it right with plastic !!!!

The title of your post should be "Which material sounds less". To that end and given the 3 choices it is MDF hands down.

My DIY JBL-4350 has two 15" woofers, each in a 5 cu. ft. enclosure, for their enclosure I used two layers, 3/4" birch plywood on the outside glued to 3/4" particle board on the inside, that is 1 1/2" total thickness, it is heavy but as solid and resonant free that is practically possible, this enclosure does not require any internal bracing.

I would say that because MDF is heavier then Plywood it is going to have a lower resonance point, meaning the MDF will store vibrations longer, much like a heat sink, and release it slowly causing a smearing of the imaging. Then by bracing in an effort to eliminate resonance mass is added, which again lowered the resonance point. Imaging ringing the bell in the tower of London as compared to the bell on a bicycle. Lots of energy off the back of the driver is headed to the cabinet.

Harbeth still takes the light approach with success. Not sure about their 40 series however, that may be to big a woofer.