What is the best G-S???????

In terms of raw power the most powerful greatsword is King Artillart Sword, it has 1488 attack, white sharpness and -15 affinity, the affinity isn't a problem if you have Art Of Unsheathing. Its made from Diablos materials so you'll have to farm a lot of Diablos to make it.
Gaoren Token is possibly better. It has 1440 attack, white sharpness and no negative affinity. It also gives 30+ defense boost. It requires Shen Gaoren materials to make but if you have the right skills, King Artillart Sword has the most damage.

How would i get a King Artillart Sword?

If you have Sharpness+1 and Art of Unsheathing, wouldn't the Ukanlos Destructor be much stronger? 1680 raw, 120 ice, blue->(white) sharpness and -30% affinity.

The reason Ukanlos Destructor doesn't do as much damage as King Artillart Sword even with sharpness+1 and Art of Unsheathing is that when Ukanlos Destructor is paired with sharpness+1 it has white sharpness, King Artillart Sword with sharpness+1 has purple sharpness. Purple sharpness massively increases the damage inflicted by your weapon. If Ukanlos Destructor had white sharpness, not blue sharpness to begin with it would definitely be the most powerful weapon. Also bear in mind that the Ice element doesn't mean much. Greatswords aren't good at dishing out elemental damage.