I dunno how others feel about it, but The Seven Deadly Sins.
I had high hopes for it. It had everything I look for in an anime: magic and fantasy, mystery, fight scenes (okay, I did enjoy some of the fight scenes), good animation, a caste of decently differentiated characters...but then...I know they just dropped the ball on all my expectations. I think I made it to the beginning of season three and was just kinda..."meh." I can slog through just about anything.
The pig's voice grated on my nerves, too.
The second worse anime I ever watched was Fushigi Yûgi: Eikoden. I had to order this online back in the day to see it, and I was so irritated by how stupid the story was, I just got rid of it XD I wasn't really a fan of even the second season (or the second part of the manga series), but I have a high tolerance for...uh...I guess silly stories, and could find a tiny bit of entertainment from it. But not this third season that wasn't part of the manga series.