Well compared to the pizza you'll get just about anywhere in Italy, none of them exactly cut the mustard, but, in order of preference:
Pizza Express - decent enough pizza with no gimmicky toppings (aside from the ever popular pineapple, which, in reality, isn't that much of a crime). Pleasant, if slightly soulless eating environment, good salads and desserts are a bonus.
Dominos - provided you stay away from the aforementioned gimmicks (bbq sauce topping, ridiculous number of crust varieties, nasty, artificial dipping sauces) you can get a passable pizza here.
Pizza Hut - nasty, disgusting crap, stay well away.