Well, for those who like to swing their weapons around... the holy grail could be one that seems to just come alive in their hand. Like, the warrior who had this years and years before me must have appreciated a grip-size and balance similar to my tastes....
With blades there are many nice pamor blades, amazing wootz and twistcore, lovely ivory, gold, swassa, silver, etc... interesting provenances...
Just as with guns there are the "holy grails" of finely crafted damscus barrels, extremely handsome walnut stocks, and silverwork done on the locks - all the way to high-tech plastics, tactical stocks and accessories, extremely rugged and powerful scopes, cold hammer forged chrome lined barrels... Vasili Zaitsev's rifle, or your Grandfather's Tompson, or even George Washington's own pistol....
or even with say, bows... some preferring composite bows made of horn, bone, and wood... others preferring a simple self-bow made of yew or Osage... and still others feel their holy grail is a modern made synthetc bow with all the pulleys and sights and stuff...
But sometimes a weapon will just feel "meant to be" in your hand. Not just "oh that's a nice blade" but, "wow, I can't believe this was not custom made for my hand and my preferences". That, to me, is a holy grail.