Mine was Noir (2000), got introduced to it when I walked in on my dad watching it and kept watching anime since. It's a real cool show about two assassins that make a pact with one another to kill the other once they discover the other's true identity. If you like shows with lots of stylish violence, metal gear style characters and all the "vintage" sort of anime styles from the era it'll be up your alley. It also has a kickass soundtrack to boot, this was one of my standout favourites.. seriously give it a watch!
