I really like Deep Cut Studio's mats. Would recommend either the mousepad or the cloth, as the PVC has a slight glossy sheen to it that looks a little weird. Mousepad is sturdier and higher quality, but the cloth is cheaper and easier to transport and store.
I've got their cobblestone mat in 4x4 for normal Mordheim, which is great, but their new Medieval Ruins would also work for classic. For BtB almost anything goes, since you'll be playing in the Mountains of Mourn, in Cathayan towns, deep into the mutating Chaos Wastes, and everything in between. I could see any of the following working for various scenarios: Frost-grave, Plains, Swamp, Wasteland, Inferno, Badlands, Alien Planet, Red Planet, Tundra, Muddy Field, Winter, Snow Fields, Asteroid.
Really, pick based on what would match the terrain you've got. If you've got a wide variety of terrain (and a big enough budget) it would be cool to get one classic Mordheim style mat, one snowy one for the mountains, blizzard scenarios, one generic outdoors (Plains, Tundra, Muddy Field), and one more exotic one for the Wastes. If you're only going to get one I'd stick with one of the generic outdoors.