Everything below excludes the great lakes and their tribs, and speaks to wild/fingerling stocked fish only. If monsters come from a hatchery, they can exist anywhere, so it's kind of beside the point to talk about. We're looking at places that grow em.
Have to agree, the Allegheny below Kinzua would rank right up there as far as places to catch them in PA.
I catch a one or two in the 17-20 inch class every year. Such fish do exist in surprisingly small streams, and can be caught in bigger, richer streams too. The difference is that they're seemingly a true anomaly in the samll streams, i.e. there aren't many in the 15-20" range. You have the small and medium guys, and then the monster. In the larger streams, like Penns, well, you can sneak up on that number. You get tons in the 12-15" class, and a fair number in the 16-20" class too. You occasionally see one that probably pushes 21, 22, maybe 23 inches. Rare to find anything higher than that.
My largest wild trout I've brought to hand in my life was 22". It was actually in the Tully of all places. Off the mouth of Cacoosing, and yeah, it was indeed wild. I've also broken the 20" mark (wild) on Spring Creek and BFC. But I'm probably approaching double digits as far as fish in the 17-20" range from a variety of locals.
I can't say that I've ever even seen an honest 30 incher. That doesn't mean I don't think they exist. If I was searching, and I'm not, the D and the Allegheny, maybe the Lehigh, would be high on my list.
But don't overlook the borderline marginal waters. Pine, middle Clarion, etc. Such streams have plenty of food, but few trout on account of temperatures. Only the dominant fish get the cool water refuges and make it. The rest of the year they have the run of things.
SC limestoners need to be mentioned as well. They have a history of turning up beasts.