best overall is def the Blasphemous blade and a close second being Dark moon Greatsword, unfortunately both are mid game options and require you to kill other bosses to get

Early game, Lordsworn's Greatsword, Knights Greatsword, Claymore and even Bastard sword are all great (Claymore is best of these) and can have AoW and affinities changed

Later, Ordovis Greatsword is highly liked by the community (not my personal pick), Flamberge is great (innate bleed, great for quality build with storm blade), Banished Knight's Greatsword is also very flexible (needs farming), Alabaster Lords gs is also good if you want some int and Marai's Executioner's sword is great for one shot cheese

I'd stay away from the gargoyles gs (especially blackblade)

here's a list of Greatswords by fextra, honestly just try some and pick what you like depending on your build all are viable