Coolest? Or most powerful?
For coolest, I'd have to say the Seventh Sword from Dave Duncan's trilogy.
To give some background, in a low-tech (as in, bronze age barely headed into the iron age) culture, a grand master swordsmith, Chioxin (IIRC), bargained with the chief goddess of the world. In return for making seven masterpiece swords, he gained an additional seven years of life.
Each sword had a different gemstone in its pommel, associated with the different rankings within each craft. (Novice, Apprentice, Craftsman, something, something, Honorable, Lord. White, Yellow, Brown, Orange, Red, Green, and Blue were the colors associated with each rank. Note that, while there's a cut-off, more or less, between each rank, there's no top to Seventh rank...) The pommel and handguard were each shaped to a different mythological monster. On the blades, on one side, you see seven men fighting seven different monsters--no pose repeated in any of the blades. On the other side, you see seven women feeding and pampering the same seven monsters--again, no pose repeated.
The seventh sword, probably a bastard sword or greatsword due to its length (it could only be wielded by a giant of a man--they fought one-handed, with no armor or shields), had a sapphire at the tip of its pommel. It had a griffin for its designation. The griffin stood for a motto, "Justice tempered by Mercy." It was a sword worthy of kings, a masterpiece of art, but perfectly balanced for a weapon.
(There was also one with a basilisk sword, which stood for, "Power wisely used." This one, however, had shattered in a battle, and was only available as the hilt and part of the blade.)
Okay, rambling done.