One hand and shield (to use ESO terms). It's the most interesting way to fight in the game, and the higher-level block perks let you do some cool stuff. Dragonform (or whatever) shout + Slow Time shout + tree sap potions to slow time + blockade runner = me charging a group of enemies, launching them all into the air, and still have enough time to pull out your crossbow and shoot them all dead before they hit the ground. That was one of the cool parts of Skyrim combat: the game didn't create epic combos, but higher-level perks and pure chance create some crazy combat scenes.