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

A combination of materials are good - on most of our speakers we use a solid hardwood front up to 2” thick depending on model - with a heavily braced rear enclosure typically made from 1” thick MDF 

As with anything implementation is equally important

Good Listening

Peter

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.