Hellgrammites! There's no doubt in my mind that they are the best live bait ever invented. These are the larval stage of the giant and supremely ugly Dobson Fly. They are also called "toe biters", "conniption bugs", and "go-devils". Hellgrammites are the top inverterbrate predator in the waters they live in, they are voracious predators. These are the bengal tigers of their world. They don't swim well but they can cling better than a klingon. They generally live in rivers and streams with swift current and rocky bottoms. Their habitat extends pretty much all across the 48 states. Not sure about Alaska or Hawaii.
Be advised; these bugs can, and will, bite the hell out of you. They have short pincers that easily break skin. On the other hand, it's easy enough to avoid getting bitten if you are careful to pick them up just behind the head. There is a tough collar just behind the head that is a good place to put a hook under and back out. I've caught several fish with just one hellgrammite due to the toughness of that collar. It also allows them to more or less hang in a natural position.
They grow two to three inches long, and live under rocks. The most effective way I know of for catching them takes two people. This works best in water that has good current. One person stands down stream of the suspect rock, and holds a piece of window screen downstream of the rock and tight against the river bottom. The other person stands upstream of the rock and slowly lifts it up while looking for the hellgrammite. If he can, he picks it off the rock. Many times though the hellgrammite will let go of the rock and take off with the current - hence the screen to catch the "runners".
Every species of fish loves hellgrammites the way I love cold beer. You never know what you'll catch on these fellas, but you will catch fish. Their neck collars are so tough that they can be cast with a flyrod, you have to cast gently, but you can do it and have a lot of fun with it. Some day someone is going to figure out how to raise them commercially and get rich as sin. I'd be more than willing to pay for a bucket of them every now and then. There are some places where you can buy them from bait stores, but they are few and far between. I've occasionally seen them on sale on craigslist for around $8 per dozen. Good luck on finding them for sale though - you'll probably have to catch your own. If you use these for bait you won't need luck to catch fish. Not in my experience anyway. It's as sure a thing as fishing offers.