I hate 90% of all anime that feature High School girls doing stupid things that are supposed to be funny but aren't. This includes:
-Lucky Star
-All of those other anime that are just like what I described but with forgettable names
-K-ON!(!)
Haruhi is freaking awesome, though. Same with School Rumble.
On a more serious note, Legendz > Pokemon. As for the worst anime I've ever seen?
The newest season of Bakugan. Over the years they've slowly but surely removed anything even remotely interesting about the show, resulting in a show that is arguably so bad it's good because it's now just a bunch of kids with horrible, horrible voice acting yelling at each other and hoping that they somehow win. I watch it for laughs, because it's just that bad.
In second comes the latest season of Beyblade, which is pretty similar, but it doesn't quite reach "so bad it's good" territory.