Plywood is the best stuff really. "birch ply" is considered the cabinet grade stuff and is usually handled a lot nicer than wall-grade which is usually stacked vertical not long after drying.
The difference being your cabinet grade stuff is nice and flat when you start working it rather than the cheapo stuff which is usually warped because the glue has not been allowed to set properly.
Ply will work better for a monitor mount than chipboard and also is easier to groove using a router.
MDF is great stuff for shaping with the router and cuts like butter but it's basically sawdust and glue. It swells if it gets wet, even if it's been primed, coated and then sealed unless you put about 9 coats on and a wax sealant - and then you're getting to furniture making.
All grained wood needs priming first, preferable to use a spray system if possible to get it even - the better your primer coat, the better your finish coat.
Hope that helps.