Alright I should give some info now on how I ranked things:
Pepperoni - not much to say here, it's the most popular topping in the world for a reason.
Onion - the reason this is in S tier is not because it's a star ingredient, but because it is the most powerful support topping there is. There isn't a single topping combination that isn't made better by adding the right kind of onion to it.
Sausage - Second fiddle to pepperoni, but nearly as popular due to it making an excellent pair with a variety of vegetables as well as other meats. There are also a few kinds of sausage which adds even more versatility. It can even star on its own.
Green Pepper - green pepper is to onion as sausage is to pepperoni. That is, an extremely good support topping that can go with almost anything.
Extra Cheese - who doesn't love more cheese?
Mushroom - ends up here due to sheer popularity, despite it being a relatively mellow flavor.
Meatball - had this here instead of ground beef because a good meatball includes lamb, veal, and pork as well. It can be bulky, but properly sliced it is probably the tastiest meat on the menu and it can carry a pizza.
Prosciutto - not the most versatile meat, but it has extremely high taste rivaling that of the meatball.
Spinach - excellent vegetable for complementing a heavy meat pizza, also has good synergy with other vegetables. Its flavor can be somewhat bitter to some, and it needs to be cooked a particular way to avoid making the pizza soggy, which is why it's not ranked higher.
Bacon - tasty enough to get it into B, but it's honestly better off a pizza than on it
Ham - a less distinct flavor than bacon, ham makes up for it with more versatility in topping combinations.
Red Bell peppers - solid flavor, but more easily buried among other toppings and does not provide strong enough support to be ranked higher
Chicken - this will probably be moved to A, since chicken is the only meat that I feel can be used in not just any topping combination, but with any specialty sauce as well.
Banana pepper - crisp and good on pizzas where you want a slightly sweeter option, but its options are limited and thus its rank.
Tomato - good in a few specific applications, but too mild to really work outside of them.
Feta Cheese - here because of its distinct flavor and usage in certain niche combinations
Green olive - really doesn't belong on pizza, but it's ok I guess
Jalapeño - flavor is too strong so it doesn't really synergize well with other toppings, and it doesn't really taste that great on its own either.
Ground beef - honestly pretty boring for a meat, lacks the flavor of the seasoned pork products and the versatility of chicken.
Anchovy - too salty, but not horrendous
Black olive - you're lucky I even tiered this because they are vile.
