This question gets asked all the time (I even asked it myself a little whie ago to get some input before I started my next project.) You'll get answers stating that any of several different types of material is the best.
I've come to the conclusion that all of the major choices are viable and which you use really comes down to personal choice.
Particle Board: Inexpensive, medium weight. Needs priming/filling & sanding if you're going to paint it. Should have a 2x2 frame in the lower section to support the monitor and 1x1 real wood (1x2 or whatever) stringers for attaching pieces together. Many original cabinets were made from Particle Board, so obviously it's a useable material. Don't get it wet.
MDF: inexpensive, HEAVY. Paintable without much prep. Messiest to work with.
Plywood: Most expensive of the 3, lightest weight, overall the strongest. A good birch or ash faced ply won't need much prep before painting.
Personally, I'd put a 2x2 frame in the lower half regardless of what material I'm using. Especially if you're using a big-ass monitor or TV.
I'm using particle board for the two cabinets I'm currently building. The best compromise between weight & cost. I've seen plenty of 20 year old particle board cabinets that are still in fine shape (I own one, in fact), so I'm satisfied about its lasting power.
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