Pizza

It sort of depends on the pizza but many Poskim today rule that pizza, even one slice is Hamotzi. This is since pizza has turned into more a meal than a snack as it used to be. Unless the pizza dough is made with more oil and milk than water because then you would make Mezonot on one piece.
The Gemara in Brachot says:
The Gemara says that food eaten during a meal is exempted by the bracha of Hamotzi, except for foods which are not part of the meal, those you need to make a bracha on. So fruits and sweets which are not considered as main courses and dessert are not included in the meal bracha of Hamotzi. The other type of food which one is required to make bracha on is Pat Haba Bkisnin.
it now relates that Rav Huna ate thirteen substantially sized, sweetened loaves, three loaves per ‘kav’ of flour, and he did not recite Grace after Meals because they were not genuine bread. Rav Naḥman said to him: That is hunger. One does not typically eat that much merely as dessert. Rather, over anything which is substantial enough to satiate and others base a meal upon it, one must recite Grace after Meals.

The Gemara understands Pat Haba Bkisnin is bread which you do not eat as the main part of a meal. It is made for desserts and snacks. If one would eat enough to satiate himself then he would have to wash and say Bircat Hamazon. So it is ruled in Shulchan Aruch 168:4. The Poskim disagree how much Pat Haba Bkisnin needs to be eaten to be considered a meal. The Mishna Berura says it is at least four eggs worth, the Aruch Hashulchan says it is dependent on how much own would eat as a part of a main course.
The Poskim are less sure about if the bread is filled with cheese or meat, savory fillings. Do we say it is just like Pt Haba Bkisnin, filled bread. Or does the halacha differ and since it is filled with meat or cheese is it something people eat for a meal and then it would need Hamotzi.
The Shulchan Aruch 16:20 writes: Pashtida, with fish, cheese, or meat is Hamotzi and you need to say Bircat Hamazon.
The Taz asks why is this different than regular Pat Haba Bkisnin which you need to eat an amount of a Seudah to say Hamotzi? He answers that it is the same thing just you might have thought the bread part is subordinate to the meat and you would not make a Hamotzi even if you ate an amount for a meal.
The Magen Avraham disagrees and he understands simply that it is Hamotzi and different than Pat Haba BKisnin. Pat Haba Bkisnin is made to be a snack or dessert so you need to show you are making it into a meal to be obligated to wash. He quotes the Sefer Tanya which says that you still need to make a meal out of it as well.
The Mishna Berura leans towards the Psak of the Magen Avraham but qualifies it that it depends on how much meat there is. If there is some meat or cheese chunks mixed in the dough then it is similar to Pat Haba Bkisnin, since no one would make a meal out of it. However if it is something that had a lot of filling then it has the Halacha of regular bread and you say Hamotzi.
So it would seem the criteria for making Hamotzi is whether Pashtida is made as meal. So the question is Pizza.
Since many people eat Pizza for a meal, the fact that it has cheese on top does not change it into Mezonot. Instead one should say Hamotzi one piece .
It depends on what a slice of Pizza is eaten as. Since most people eat pizza for lunch or dinner not just a snack one must say Hamotzi on one slice. However if you live in place where pizza is mainly snack then it would have the halacha of Pat Haba Bkisnin. tis mad e a snack food then you would need to eat enough to make a meal out of it.
That is if the dough is a regular dough but if the dough itself is made with mostly juice or milk and you can taste it when you eat it then it would also revert to just being Pat Haba Bkisnin.