Can be anything from any source, movies, books, comics, video games, manga, anime etc.
Pic related is mine, the dragonslayer from berserk. I like it because it's simple. It's just a big fucking sword, no fancy bells and whistles like enchantments or spells.
Second runner-up is sting from lord of the rings, I like that it glows blue when orcs are around and it can be used in the dark as a light source.
Bleach had (has? I hear it's back on again now) some really great ones. Basically every weapon has multiple forms where it gets cooler and cooler, often culminating in a battlefield filling sentient monster.
Abarai Renji has Zabimaru, a hooked whip sword that transforms into a flying bone dragon.
Kuchiki Byakuya has Senbonzakura, which looks just like a normal katana but its special form scatters into thousands of tiny blades which are only visible as cherry blossom petals blowing in the wind from the light glinting off them.
Ichigo, since he's the protagonist, has an extra special sword that gets smaller instead of bigger in its special form, which concentrates his power and makes him extremely fast and strong. It's normally like Cloud's sword but shrinks down to become like Sephiroth's but all black.