Tidus' Caladbolg / Ultima Weapon is my favorite.
When I first got it I became obsessed because of how much I liked its design. Its name is derived from the Irish counterpart of Excalibur. One of its nicknames means hard lightning, which is fitting because that's what it looks like: A thunderbolt in the shape of a sword. God it's so cool.
My second favorite is Auron's Katana.
It's simple, yet stylized. It's almost elegant in its simplicity. It's a big slab of steel and barely even a sword, which makes it the perfect weapon for Auron, who is basically a wild brute, despite formerly having been a warrior monk. I'm trying desperately to why I Iike it but I'm falling short of ways to do so without sounding like a pretentious git.
And although it's not a weapon you use, I really like the design for Yojimbo's Zanmato.