When discussing swords here, I note that Fingolfin's sword Ringil is never mentioned..despite the fact that it was the only one to not only combat Morgoth, but actually wound him severely and permenantly.  This makes Ringil, IMO, the most accomplished sword in Tolkien's mythology and possibly the most powerful.  Remember, the other swords were used against equal or lesser beings..Gandalf was the Balrog's equal anyway, Turin, thougha great warrior and killed the dragon while it was sleeping and not in actual combat, Isuldur cut off the ring from an unconscious Sauron (apparantly from wrestling with Gilgilad and Elendil and not sword wounds).  Ringil did its thing against the most powerful thing to exist or ever will existin the mythology outside of Illuvitar.