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

Material "X" and material "Z". Wilson Audio.

Baltic Birch ply -Russian birch ply 1-1.5inches thick on the front baffle 
1inches everywhere else, 2inches on the platform braces acoustic glues 
and real wool felt on top drivers , back baffle ,sides
acoustical foam on Bass. The Balticvariety ply like a laminate absorbs sound well ,and Baltic Birch from a colder variety climate the wood is denser . And like a wood instrument sounds even more musical when done right , time consuming ,and more expensive 
but worth it .Justgo to his site he builds  customspeakers around the world well into the $$ 1,000s.

i have made my own and worked with great speaker builders such as Tony Gee, who does all themajor Capacitor Xover reviews
under Humble home made Hifi . I learned a lot from Tony
10 years ago when I ran a Audiostore in Europe.

+1 peter.
I've wanted to play with composite sandwiches, as well as using layers of the same materials with different cut-outs to prevent panel length resonances.

I am quite keen on Panzerholz, a very dense, very dead German beech plywood with elastic properties. Costly to experiment with, though - $1000 sheet.

MDF and plywood are okay. HDF is better. Wilson uses variants of quartz and Corian countertop material. 

All sorts of composites and plastics can be ordered in sheet form from McMaster in various thickness. Methacrylate acrylic and phenol works great.

Bracing is very important. Isodamp is amazing stuff.

I would stay away from aluminum. Goldmund, Magico, and YG lost their minds. It's the worst material for a speaker enclosure.