I don't think I can narrow down a favorite! I love salami, pepperoni, mushrooms, olives (black or green), pepperoncini, pretty much any cheese, green peppers, basil, tomatoes, most veggies...
Least favorites: Italian sausage, anchovy, onions. I also dislike "theme flavored" pizzas, like bbq, taco or cheeseburger. (I like Hawaiian though!)