Lately, here are some of my favorite things to stuff peppers with:
* Cheese (of course; it doesn't have to be a special hard-to-find kind of cheese; the peppers make it squeakier and tastier; cheddar is great; so is mozarella; so string cheese, which is also a kind of mozarella; even if all you use it cheese, it will be good)
* Tortillas! These are really good in stuffed peppers, and they go well with cheese. Just tear them up and stuff them in with the rest of your things.
* Tomatoes. If your peppers can hold moisture like a bowl, tomatoes actually taste really good in stuffed peppers.
* Onions. use them, but don't overdo or underdo it.
* Butter
* Olive oil
* Salt
* Pepper

Cottage cheese is okay. I don't recommend it over the above ingredients, however.

I figured I'd start this thread since pepper stuffing ingredients that taste extra good aren't necessarily obvious. Please list more.

A sardine mixture of bread crumbs chopped onions and hot peppers and so on.

Taco meat (meaning ground beef with taco seasoning), plain or mixed with corn kernels or rice, with or without cheese.
Black beans, corn kernels, onions, garlic, possibly jalapeno or poblano pepper, with or without cheese.
Spanish rice (rice cooked with tomatoes and onions) or Portuguese rice (with Portuguese-style sausage/chorizo.
Rice and beans.
Rice with tomatoes and eggplant and Greek spices, maybe a little parmesan.
Tater tots or hash browns and cheese, or cubed potatoes, onions and cheese. Maybe add egg near the end.

Of course, the only way to actually get rid of peppers is to stuff them with more peppers, otherwise you hardly put a dent in the pepper stash.

Around here, most anything could end up in a pepper ! One fav is a mix of breakfast sausage mixed up with finely diced veggies rice and some tomato sauce. Eggplant, diced up squash, cabbage or dabs of leftover brussell sprouts along with onions and cheeses have all been in peppers before for a lunch or dinner. Toasted seasoned panko on top for some crunchies.

The last time I made the peppers stuffed with sardines it wasn't your average sweet peppers.
They were manzano peppers.
Think small thick walled fruity but very hot bell pepper.

Here are some stuffed Blot peppers (the ripe ones), and some green bells (probably some small/immature Canary Bell peppers). The Blot peppers are cut open because I saved seeds.

Big green, nearly-ripe Canary Bell peppers seem to be my favorite flavor-wise for stuffing, this year. This surprised me, since by past experience, I figured the fully ripe ones with the most sugar would taste the best stuffed. The small/immature ones don't have nearly as much flavor as the big nearly ripe ones, but they're certainly edible.

I love the bitter taste of peppers this time of year.

I like to put sausage and rice and make stuff peppers