Well, since the blight has crashed smallmouth populations in the Susquehanna drainage this isn't true anymore. But up until just a little while ago, I'd have said the Connenoguinett in Camp Hill. If the stream were full of trout the way it was smallmouth, every flyfishing magazine in the country would list it as a stream to fish before you die.
Not a really beautiful stream... It ran through people's backyards... and for that reason, access was a beast. But the hex hatch was a marvel of nature. And there were caddis and crayfish in epic numbers. The wading wasn't hard at all either. I think the key was, there were a number of blue ribbon limestone streams emptying into it. The Letort, Green Spring, Big Spring and all the others. It was a real gem in it's day.