Worst? Elfen Lied. I felt Elfen Lied to by everyone who ever told me to watch it. To this day I can't figure out who the target audience for that anime is. Good on ya if you felt your life was changed by it, but perhaps some soul searching is in order, because I can't imagine that anime changing anyone for the better. I say this because I actually saw comments like that all the damn time.
All that love for a show that was a blatantly lolicon anime that wanted so badly to be taken seriously, but still wanted to be a silly hentai. I guess this is what they mean when they say "you want to have your cake and eat it too." -- It just couldn't decide whether it wanted to be precocious or gloomy, so it just settled on awkwardly disturbing. This show so graphically handled child abuse, rape and gratuitous nudity that you couldn't in good faith tell me that you'd watch it in front of someone that you wanted to have any semblance of respect for you. I'm sorry, but no, suddenly growing your obviously underaged characters breasts too large for their age whenever they become (often inexplicably) naked does not mean that you have cleverly avoided being racy.
What's more, the show went so over the top with violence that by the end, any suffering on the part of the characters had no more gravity and thereby laid waste to what could have otherwise been touching moments...If not for the fact that I'd been so convinced that it would somehow get better at some point I would have never seen it through to the end...Not even for the admittedly beautiful background art and soundtrack... On that note, why was the character art and animation so shitty when the background art was so drop-dead-gorgeous? It's like they got someone with no knowledge whatsoever of human anatomy to handle the characters. Now, if I was going to make my underage characters naked THAT DAMN OFTEN I would have made sure to hire a team of animators that knew how to draw!
Whew, I feel better now.
That was actually really, really easy. I don't usually watch anime that I'm not entirely sure will be good, so I haven't really seen any bad ones asides from the one mentioned above. Sure, I miss out on a lot of good stuff with that attitude, but I am not a big enough fan of anime to care much. Usually it just means I'm really late to the party on the stuff that's good. That's more than worth it to avoid the worst of the worst.