Probably the Draconic Staff of Power. If you don't want to put yourself at as much risk of blowing up your entire base, use the Draconic Sword.
The sword deals ~19 damage to EVERYTHING within several blocks of whatever you hit, and another 10? damage to the mob you directly hit if it has high enough health. Then, if you put the Vampirism enchant from Thaumic Tinkerer on it, it heals you for EACH mob it his. One swing in a swarm of zombies can instantly refill your entire health bar.
I shall make one immediately