The Soul Reaver. In Legacy of Kain: Blood Omen, it's the most powerful weapon in the anti-hero's arsenal, using up mana energy quick, but making up for it by killing any standard enemy in one hit. And come the sequel, Soul Reaver, the sword shatters upon making contact with new protagonist Raziel, and now serves as a weapon that you are equipped with at full health, rewarding players who've mastered the combat.
Nero's sword, the Red Queen from Devil May Cry 4, is another good sword: The customised Order sword allows its user to rev it up for a truly damaging attack, and is so much fun to string together combos with.