3/4" or 5/8" (I am using T-Molding)
I'd go 3/4" for a full size.
If you use 5/8" on anything other than a bartop, it will look too small.
MDF or Particle Board
MDF is better than particle board for almost any application.
It is nicer to work with, and doesn't fall apart like particle board.
I personally used plywood because it holds up even better than MDF.
Whats the best way to create the door for the back panel?
I used 1/4" plywood for all the non-essential pieces on mine (top/back/speaker housing/etc...).