It depends on what you want, really. Everything I ever made was on mdf or cross veneer plywood because of the strength and warp resistance. For me either one of those is a must. After that it's how portable you want it and what surfaces you'll be setting up on. A wide flat surface means you can have 6x 2ft square tiles which has a lot of combinations as you can also rotate tiles. 3x 2ftx4ft is less portable but still practical and has the bonus of being able to be set up on narrower tables. 2ftx4ft tiles are also, oddly, easier to store in a game room since you can lean them up. I'd say the above is the starting point to what your board to be
After you've decided on the tile sizing modelling the table will be on your mind. Obviously, the flatter and sfter the surface is the less it will wear so a 18mm thick mdf board with a greassmatt glued on will be very hardwearing and you can use terrain pieces so it's the ultimate blank canvas for wargaming. You can also flock, glue sand or use textured pain and artex. Flat unlandscaped boards also can be textured on both sides so you can flip them over bit the can feel a bit lacklustre Foamcore/insulation foam carved and textured likely makes some of the most impressive looking tables since you can have hieght and cut depth but the issue is that it's easy to knock a corner off so less suitable for portable boards unless you're really careful. If it's a permanant set up then it's better for that. Toavoid knocking the edges you can also box the sides in with a frame you can drop the boards into to make a permanant table (sides stop you losing dice) and you can make a lid you can flip over to have two tables in one. The box idea is also good for fully modelled boards with buildings, etc.
No hard and fast rules on what materials and techniques to use but I like insulation foam since there's always a skip full of damaged ones outside a house somewhere in your town.
I havent a phot of the whole board...but this gives you an idea of the surface painting. It needs more work but is a start. I got one big sheet of the Mdf 12mm(?) cut in two. Put two battons down each board so it sits on the dinner table without being tippable.
Its actually too big at the moment...but we can grow into it. Its at its best when the lads mates come round and theres loads of 40k on it. For me and my mate one half is enough to play on the other becomes a dumping ground for rules and the 'dead pile' (last time all mine!)
My original table and boards, now on loan elsewhere, was done the old fashioned way with 24 inch square, 2 inch deep boards. It worked but it was never really used to its full 8 × 6 glory and the boards were a bugger to store.
My current boards are 24 × 12" and are made of 18mm MDF which is, in retrospect, rather too thick and heavy, 12mm would be perfectly adequate. The boards were either one piece or for those with recessed features such as rivers made from two 9mm pieces of MDF. They are much, much easier to store as they do not have any raised features so stack nicely on a (very strong) shelf. I have access to my father's workshop which includes a bandsaw which is pretty nearly essential for doing rivers the way I have done them. The boards themselves I got cut in B & Q which worked well, was quick and neat, more accurate than I could have managed and made them easier to get home but did leave me with a bad case of saw envy. There are another couple of sheets of MDF waiting for use but these might get re-purposed for Great War trench sections, retaining the 24 × 12"footprint but adding depth.
Hiding in a corner are some experimental boards made 10 inch or 25cm square from 6mm MDF. They are even easier to store,in A4 paper boxes, and are almost portable. I think they would be worthwhile developing though 6mm might not be thick enough to allow recessed features. At the time I was considering going metric and using boards that were multiples of 25 or 50cm, 6 feet is just a bit too wide but 150cm would be about perfect for width, and if I was starting again this would get serious consideration.
A legacy of another daft idea somewhere along the line is a series of 12 inch square 6mm thick urban boards. These are just building but originally were to have road sections as well in 12 × 6 inch sections (with additional 6 inch square intersections) that could slot in between the blocks. This is still my standard for urban terrain, a legacy of 40K. Some buildings are stand alone but others were designed to be part of a block, two or more squares across.