Well, I'm going to go with the serpent shield... Not because it's the best but maybe it's one of the easiest to get early game.

You can get one with a finewood bow and a karve maybe even a raft. Lure a sea serpent to shore, jump off the boat and be quick and kill it. (I had to build a couple of ladders up to dive deep enough to get all the scales.)

Idk what it does late game because I haven't killed Bonemass yet with it, but neither a Draugar or troll or wolf can beat it. And the knockback makes fighting mobs a breeze and funny too. If you can get them by a cliff...