There really is no "worst" hero. There's a low tier of heroes, but within the tier, which one is worst depends entirely on the situation.
If I had to choose though, I'd go with techies or pudge.
I need to hear this one out.
If he's not in melee range of an enemy, all he can do is hook. Hook requires a clear shot of someone, and even the best players miss sometimes. His one disable is his ultimate, and it's also channeling, thus easily interrupted. He is a subpar ganker, a mediocre tank, and pretty useless late game.
He's definitely a fun hero, though.