Movies like Requiem for a Dream, or books like The Road are the kind of stories where once is enough. Anime is like that as well, with series - that for some reason or another - only warrant a single viewing.
Usually it's explicit content that limits an anime's re-watch value. However, while that may be the case for a few on here, it isn't for all of them. Some are included for the sole fact that sitting through them once is a chore in itself, while others are the type of experience that are too distressing to even think about returning to.
Maybe it's the violence that turned you off, or that you were left feeling emotionally drained after the last episode and don't want to feel like that again.
Just as there are multiple reasons for re-watching some series, there are just as many for not re-watching others. Consider this list as a mix of the good, the bad, and the ugly of the anime world.
With all that out of the way, let's get this party started with some anime series you only watch once.