Anything perpendicular to the gravity vector.

I really like playing on my neighbor's billiards table (felt).

I was actually talking to my wife last week about how it would be cool to have a magnetized wall, and then play X-wing on the wall, with magnetic bases...

a little like this you mean?

Deepcut studios mat. But placed on magnetic table V2. Celotex (insulation) board.....Mag sheet glued to that and then the vinyl mat laid on top, not glued. Bases of mini's have been covered with card, painted black and a steel sheet paper layer added.

Picking up the celotex board for V2 on Sunday as V1 didn't work out as intended (Never listen to an older brother who is in to DIY)

5 naked super models, face up and as close together as possible, lying on a table made of chilled beer kegs...

Presumably what you meant to say was :

"Apart from the awesome magnetic game mats a certain poster has for sale on EBay, what is the NEXT best surface to play on?"

Very well then, the NEXT best surface to play on is the following rug, available at Lowe's. It gives you the pleasure of feeling like you're flying ships around your neighborhood, without the crazy side effect of people seeing you running around town pretending to fly spaceships in your hands all over your actual neighborhood. Plus, it offers a nice non-slip surface.

Okay, so its my kid's rug and it just happened to be 3' by 5', which allowed room to place our cards and tokens on one side while blowing each other's ships up over the rest of the "neighborhood."