I use or own four of the suggestions in this thread.
(1) Our primary mat was made by one of our players and donated to the gameroom. It's a stiff, glossy, gridded overlay on vinyl. The reason it's primary is that it's about 32 inches by 80 inches, which makes it long and narrow enough to use on the full-sized poker table we game on. I believe that getting this done -- as opposed to DIY, as he's the office manager of a graphics design place -- would be pretty expensive.
(2) We have several Chessex mats in two sizes. These were my preferred solution, and I still like to use them if I'm carrying stuff only a short distance. I like that they lay flat, hold ink well, yet come pretty clean.
(3) For longer trips, like to GenCon, I have 10 or so of the GameMastery flipmats (all the different settings thus far). They're very convenient to carry, and if counter-folded properly lay flat. They're not as nice to draw on as a vinyl mat, nor as large as the larger sizes.
(4) I also have easel pads, and I use them to pre-draw large and complex encounter areas, often using lots of color highlights, so they're pretty. If the encounter area is likely to see reuse, I get the sheet laminated. For example, I've done several sheets of complex tactical scale maps of the city of Sharn, with severe drops of elevations, criss-crossing bridges and skyways, towers, and so on.
As usual, what you'll want to get, singly or mix-and-match, will depend on your circumstances. I personally never got a chance to try out the Tact-Tiles, which I regret. I figured there'd be plenty of time to grab some ... so much for that.
