I can't realy pick a winner because it would largely depend on the people visiting. Some are into RPG, so I would get out the ones I know they would like (ie related to their style of play and/or character), for those that are not realy into RPG's I would pull out either the books they could relate to in other ways (ie Star Wars for a SW fan, to show them how their favourite world is described in game terms) or otherwise the ones with the best art.
If I had to pick one I guess it would have to by my DMG signed by Todd Lockwood (who drew a great picture of my character at the time, an elf thief/sorcerer, riding on the back of a dragon [it did happen to me in-game!])
Nice detail: Todd started by drawing his staple: a dragon. When I told him about my elf character and the dragon ride he promptly
added an elf character on the dragons back and asked me what color the dragon was. When I said white, he answered: "Damn, if I had known. I drew you a red" What a nice guy he turned out to be !!