I would choose sicilian with no toppings from a local place, Via Veneto; but since it’s a local mom & pop place with only one location it fails your requirement that we choose a chain. It’s second to none, and toppings only detract from it.
Fortunately, the second best around here is Franzone’s, which has two locations (which hopefully meets your chain requirement). Again, it’s so good that it requires no topping.
Pepperoni
Mushrooms.
Sad there is no poll.
Sausage, Gino’s East. Of ocourse, Nancy’s, Lou Malnatis, Giodarno’s, are also acceptable.
Nancy’s isn’t a chain though, is it?
ETA: Nevernind. Had no idea they had additional locations.
Pepperoni, but not because I consider it an essential pizza topping. I have a bunch of favorite toppings that don’t include pepperoni. But I sometimes pick up a single slice of pepperoni-only pizza at the pizza counter of a grocery where I shop. There’s something satisfying about the texture and simplicity of that particular plain cheese and pepperoni slice that makes a nice snack.
Only if it pays enough…
Looking at the OP again, I realize that I violated the requirement that plain pizza is not allowed. So I’ll say Franzone’s with extra cheese, even though it will detract from the pizza.
Pepperoni, from Mama’s Pizza, a local chain
Salami, but if that isn’t available, then pepperoni.
There needs to a “like” button. Cuz this is my response. I’m also partial to Rosati’s.
Onions from one of the local places nearby. For a small town, we have several good pizza places.
Onions or green peppers…
I’ll go with green peppers.
I think shrimp sounds good. A nice variation in texture. I mean, as long as I can have a bottle of louisiana alligator spit to squirt on it.
Gluten free from Ledo’s. Add Parmesan cheese.
It appears pepperoni is a popular choice and coincidentally I had a pepperoni pizza today with no other topping other than some extra cheese. It was good.
Pepperoni from RC’s New York Pizza (actually in Houston’s northern suburbs).
Chicago sausage (Italian sausage with fennel). Final answer.
Or green pepper-onion (it’s one topping if you truly believe 
).
Or pepperoni.
With extra cheese.
You can almost tell to a tee who the Chicagoans (or nearby denizens) are in this thread, not by the chains listed (which does give it away), but solely by the topping of choice: sausage. And not that New York-style Italian sausage on pizza which comes in slices. We’re talking bumpy lumps of Italian sausage removed from its casing.
It’s always seemed to me that pepperoni rules the single-topping universe for pizza elsewhere in the US, but sausage is the predominant favorite hereabouts, at least for the omnivores.
I’m not a Chicagoan, but I still chose sausage.
And I’m baffled by the mention of “Italian sausage with fennel”. Isn’t that just “Italian sausage”? If it doesn’t have fennel, it’s some other kind of sausage.
Yes, yes, pepperoni (or anything else sliced) is technically a kind of sausage, but that’s not what anyone means when they say “sausage”.