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.
I kinda liked the Galaxy Eater from the 50th anniversary of Doctor Who. The implication was that it had the capacity to erase whole galaxies from time, but it never got used it because it developed a conscious. It judged anyone who tried, manipulating them psychologically and temporally.
No idea what this is because I've never watched that show but it sounds cool.