How thick should the front baffle of speakers be?

The thickness of the front panel should also be determined by how wide the front is. A large 16" panel will necessitate a stronger panel material. Narrow front panels not so much. Plywood is good, better then mdf. Alon  Wolfe went to plywood on his earlier versions of the Magic monitor. 

" wheres the proof "  Simonmoon is 100% right and that has been my experience too and measurable through REW and TrueRTA but you can't post results here to prove this because no pictures allowed.
  I had a set of La Scalas that had been damaged and I decided to replace the 18mm sides with 25mm Baltic Birch sides. You don't have to believe me but the perception of much lower bass was startling. What I was actually hearing though was bass without cabinet resonance masking it and it was a real eye opener. Since then everything I build with any area at all gets 25mm wood. Real honest wood and not MDF.

" Hi, The speakers that I own (Sonist-"Concerto 4") have two inch thick poplar wood front baffles.   I talked with the designer (Randy Bankert) and he said he tried a bunch of different woods before settling with poplar as it was the best sounding to his ears. "
  That's interesting. The old Klipsch Chorus speakers used  1" poplar plywood for the front baffle and I always though that was because it was cheaper. You pick up pieces of differing types of plywood or MDF roughly the same size and tap on them and the range of sound is pretty large so what that guy says makes complete sense. Those old Chorus speakers are my favorite among the Forte, Heresy Cornwall and Chorus.

simonmoon, thank you for being the voice of reality here. Cheers.
The reason they used plywood back in the old days was that they had not developed MDF yet which is a far better inexpensive material to make loudspeakers from because of it's damping characteristics and higher mass. It is much stiffer than plywood. The old speakers were indeed musical instruments and colored beyond belief. Just listen to any Bozak B302A a speaker that was lucky in that it's colorations were euphoric as all get out. But some of the old Altecs were honky old boom boxes. Klipsch did a good job of controlling it. Higher mass and thicker stiffer walls are always an advantage however the speaker manufacturers have to balance that against shipping costs as for most it is a competitive market. People who buy Magico M7s could give a hoot.You have to pay a rigger to get the things into the house not to mention pray that your floor will support them.

You don't get doppler effects off baffle flex the baffle exists to hold transducer to support mid-bass frequency and to limit vibration levels. An over small speaker will require BSC- baffle step correction in its network to compensate if baffles undersize it is why small speakers sound thin. Also aluminum is cheaper by the ton than plywood and it is also lighter why it is used so much over ply it is not better sounding just cheaper to build ship etc.