Depends on what you want to do; it's generally accepted that out of the Antigua lines, Sentenza is mostly terrible, and Argent Peacemaker is only useful in FSC.

As for the sealed sword lines, Divine Avenger and Faust are both ok; Faust gains the ability to curse in exchange for a worse charge attack with a drawback, but it can curse monsters (not the user) on regular attacks. Divine avenger's charge attack literally has triple the damage potential of Faust's, though hitting with every possible part of it is very rare. They're both equal on terms of regular attacks.

That said, you already have a shadow damage sword, and the nightblade line isn't especially bad. I'd suggest Divine Avenger first, though you might want to grab a troika just to see if you like the 2handed sword playstyle, because the other elemental swords are still useable if you don't. Just remember that DA has a really good charge attack the troika lacks.

If you don't get the sealed sword, get Antigua and upgrade it to silversix-> Argent Peacemaker..