There are tons of posts on this topic, use the search function. The most common answer you will find is a Luan hollow core wood door. I have used ceiling tiles on a table, shimmed to be as flat as possible. I currently use a Wright Building board with is perfectly flat and covered with cork. Unfortunately, these are not available any more. You can also build a simlar board yourself. Here is an excerpt from an old thread. It's not mine but I no longer have who posted it so I cannot credit him.

1. Go to HOME DEPOT/LOWES/MENARDS what ever.

2.Get 2 Angle Irons 4' in length and 1 1/2" wide. (Hold them next to eachother to check for straightness)

3.Get 1 4'x1'x1/2" Laminated piece of shelving material. (this could be made of MDF,Chipboard etc.)

4.Get a piece of Bulletin Board Material 1/2" thick or more(This could be cork or something cork like to stick pins in. The important thing is that you have it in a big enough piece as you don't want any seams.)

5.Get 6 wood screws of a length not greater than that of the thickness of the laminated shelving in 3 above)

6.Get 3m spray adhesive. It comes in a black aeresol can and has a shelf life of infinity. (well almost that anyway) You'll use this for years to come for a plethora of projects.

7.Proceed to the checkout. Your bill should come to approx. $30. (not including the 3M Spray)

Drill 3 holes, a little larger than the diameter of your screws, in the angle irons. The ole' Mark One EYEBALL will suffice, just try to get one at each end and one in the middle of the angle irons.
Again using the ole' Mark One. Place these evenly spaced on the Shelving Material mark your holes and drill pilot holes for the screws. Attach the angle iron to the shelving material. Cut a piece of Bulletin Board material a bit larger than the shelving material. Spray each side with the 3M adhesive. Wait 30 sec. and ROLL the Bulletin Board material on to the Sheving Material.
You now have a Building Board that should be large enough for MOST beginning projects. If you need more surface make it a 2'x6' board. You can always use this little board to build your eppenages on. Some day when the Surface gets pockmarked with too many pinholes you can scrape it off the Shelving Material clean the adhesive off with Acetone and resurface it with more Bulletin Board material. The neatest thing about this way is that if your afflicted with Short Kit Attention Span Syndrome (as I am You don't have to have your workbench tied up while waiting for glue to dry. (I stopped using CyAs) Just set it aside and pull out another to continue your work.