Not exactly the worst legendary I crafted but the worse one that I crafted recently: Sherazin, Corpse Flower. it dies so fast and to bring it back it requires overcommitting your cards .seriously 4 cards in 1 turn is too much even for miracle rogue. I'd rather the condition is after playing 7 cards not 4 cards in one turn.
In Kobolds I crafted both The Last Kaleidosaur and Lynessa Sunsorrow on a whim for a deck I would end up playing maybe six times before tiring of its crappiness. To be fair I was working a dream job at the time where I was making around $300 a night. I still have them.