The best armor is the Armor of the Order, available in the last quarter of the game. It's indestructible in that it doesn't wear out, but it still provides only a finite amount of protection.
If you want invincibility, try the God Mode cheat.
The most powerful melee weapon is the sword called Mournbringer, also available near the end of the game.
If you want to be too powerful in the game, try a save where you specialize in Magic.
By the end, you'll be SO powerful.