I just got a great panini cookbook yesterday (200 Best Panini Recipes by Tiffany Collins) and the various recipes call for sourdough, focaccia, ciabatta, various rolls, etc. I think ciabatta is probably more traditional, but I'm just guessing. I've used sourdough and whole wheat with great success.

Ria

is focaccia over all others, in part because its crumb is usually more closed than ciabatta, and more open than traditional sandwich loaves. I often bake it with herbs, or rehydrated sun-dried tomatos incorporated into the dough, not merely as topping.

David G