Sup Suups -
I've used standard 8-1/2 x 11 high-quality semi-gloss, 32lb (not sure if I'm able to actually cite the exact product?), though specifically, paper meant for color copies.
The old FASA paper maps were made with something around 32lb or slightly more. While I prefer the feel of the FASA cardboard or new Catalyst (game-board style) maps, by printing on the 8-1/2x11" sheets, this size can be over-lapped in many ways to allow for many different configurations than can be done with the larger sizes.
I've thought of using a laminating machine so wet/dry erase pens could be used directly on the maps, but I've not gone down that direction, yet.