I like to use a strong mature farmhouse cheddar or a mixture of strong cheddar, smoked applewood and double gloucester (for flavour, depth and colour!).
Hehe, Mandimoo has just reminded me of my secretary's favourite cauli cheese topping - black pepper flavoured crisps, all scrunched up and sprinkled over the top of the cauli cheese, then the dish is popped back under the grill or into the hot oven.... wicked !