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

Of the same thickness, MDF is the least resonant and most neutral. Like bamboo and solid woods, birch ply boxes tend to impart a sound signature - typically a leaner, brighter sound than MDF (to my ears).

Weird that aluminum is brought up in a thread about wood derivatives. Aluminum (even "aircraft grade" alloys) will ring unless very thick and carefully braced. 

My Magicos do not ring as they are braced big time and they are very thick.

@ebm

What great speakers do you have?Good luck in this great hobby.

I design my own. Here is what I’m listening to now. The tweeter is the same as used in some Gryphon designs, the mid-woofer I’ve seen in Wilson and other speakers:


https://speakermakersjourney.blogspot.com/2017/12/snr-1-two-way-high-end-diy-monitor.html


Caps are top of the line from Clarity, resistors from Mills and copper foil inductor in the woofer. Not sure if it’s Mundorf or Jantzen. Cabinetry is from Lee Taylor

@ebm You better believe it they ring like a bell! No matter the bracing. You can't just knock on them as a test. You need a tone generator and vibration measuring tools. 

Take a look at Stereophile, every review of Magico omits this test, but all other speakers have it. There's a reason for it.