I've used wine, juice and various beers all to good effect. I am pretty sure the liquid makes the cheese less thick when heated, so it is more creamy and gooey. I made "Cheez-wiz" for cheesesteaks last weekend using water: you can use anything whose flavor complements (or doesn't detract from) your cheese, but it's going to depend completely on what exact cheese you use. I don't think there's a good way to generalize, but it's easy to test: crack open a beer and slice up some cheese.