I think that Niv-Mizzet, Parun is the best of the three because it manages to be a huge threat while also being a soft way to control what your opponents do
But if you want to optimize your money spent on your sealed product collection, buy booster boxes for the best pack-to-dollar ratio
Dartz, however, is more powerful than the three Swordsmen combined - not only is he able to repel the "Master of Dragon Soldier", a fusion of two of Kaiba's and the Pharaoh's most powerful cards, but is able to expand the full power of "The Seal of Orichalcos", by playing its 2nd and 3rd levels: "Orichalcos Deuteros" and "Orichalcos Tritos"
Ghoulcaller Gisa Though Ghoulcaller Gisa is terribly slow, it's also inexorable
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It can prevent the destruction of your Noble Knight Cards by destroying a Noble Arms equip instead