What is your favorite material for loudspeakers?

Baltic Birch layered with MDF is an excellent material for turntable plinths, particularly the Lenco. It is easy to work with, affordable, readily available and best of all, it has properties which absorb vibration. This is evident in analog playback- pace and rhythm, defined, articulate bass.
I have never used it for any other applications, i.e. speaker cabinets. But, I can only guess that it has similar qualities with regards to speaker construction which I find to be so desirable.

MDF is a pretty poor material for the application if performance is your criteria. It's popular because it's cheap and relatively easy to work with. With ply there are factors which determine it's performance, both the resin and layers used are critical ... and ply with poor quality interior layers are everywhere, much of it I've seen lately has come from Russia. But if you can find a source for quality void free Baltic Birch ply, I think it's the finest enclosure material available to the hobbyist.

As far as veneers go, there are just too many to choose a favorite. I like the high contrast, heavy grain and figuring type veneers ... the ones that jump out at you!

I've had nice results from hand rubbed finishing as well, like French Polish ... the types where you wet sand the finish oil into a slurry and let it dry to fill pores. I've also had nice results with sprayed finishes like Poly, assuming you prepare the veneer first to bring out the character.

Baltic birch plywood with veneer over it. I use MDF but only with paint or laminate. Bubinga is 1 of my favorites but I use solid bubinga over birch ply. 1 of the few hard woods that's stable. I use a lot of US sourced Cherry veneer. Since most of what I build is large veneer costs add up fast. I have a good source for great looking cherry veneer. I have tried carbon fiber, fiber glass, Corian, Plexiglas,Aluminum,HDF, MDF. Many other plys, hemp, apple, mahogany. Still to my ears quality birch ply with full veneer french polish has a more pleasing sound. The other quality plywood's are near as good as the Baltic if you double, if you go 3/4in Baltic birch ply is best. Since a 3/4in T. cab even fully braced adds much resonance to loudspeakers sound.

Are you saying you like plywood covered in a veneer? Typically when you say birch plywood, it references a layered material with a birch face veneer.

My preference is for figured veneers (fiddleback anigre, waterfall bubinga...) with a precatalyzed lacquer finish - hand rubbed appearance, not high gloss. As far as cabinet material, I've been using mdf lately, but am interested in using 12 layer plywood material in the near future.