Overall the litter problem is so much better than it used to be. I remember a time when we didn't have public trash cans and where streets were strewn with litter. Of the place I regularly used to visit, Ireland was imbelievably bad, I hope thye finally caught on.

In the wilderness it might be the case that shotguns shells and fishing tackle (depends a lot on the local fishing style and whether it involves a lot of lure loss or bad bait practices). Autoloading shotguns are a real case of out of sight out of mind. But society wide fast food beverage cans and cigarettes are the worst.