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Usually if the first 2-3 episodes don't win me over, I don't continue to watch the series so I can't say I have a least favorite.

Lucky Star. I watched like 2-5 episodes. I want my wasted time back. It was not funny and I didn't care for any of the characters. Also,

sailormoon. period.

i feel you on blood lad. it seemed like it would be such a great show; neat plot, cool black and yellow thing, etc, but actually watching it is so painful. the characters are all annoying (except wolf i like him) and that one girl w/ the backpack seems like a hxc rip off of haruko from flcl.

Soul Eater or Bleach

You could add the fact that they didn;t even finish the freaking arc for the anime. (I'm a fan of the manga and, even though the anime did it more justice than others, I can say the plot left a lot to be desired and the art really wasn't much either.)

My worst anime were:

All three Tsubasa Chronicle Movies. They were so bad my brain blocked some memories of them.
I remember one was boring ( it was called something like girl in cage), another one started from a point that didn't show up in the anime and ended the same way and seemed to exist only to establish a very, very bad appearance of the xxxHolic characters, and the other one is still too traumatising to remember.

Kiddy Girl-and
Look, Kiddy Grade is one of my favourite anime and I watched it just when a sequel had been announced. So I've been waiting and waiting and when, eventually, after loads of delays and 360 degrees turns they aired Kiddy Girl-and I was excited. Until after the first 10 seconds. Then I cried, and cried, and cried even more as I watched about 6 episodes. I still don't know if it has a plot, and am trying really hard to make myself pick it up again just to see that. To put it briefly, it's everything about an ecchi Kiddy Grade had managed to avoid: very bad jokes, panty-less moments, creepy as all shivers in Hell fanservice, making fun of anime fans, complete and utter absence of any relevant plot points, a proper explanation as to what happened to all characters or what the heck is going on, very bad art in places.

bobobo-bo bo-bobo
One episode was all it took for that thing to freak me out. It's weird even by anime standards.

Cubix
The 3d ir 2.5d or whatever it's called art was ugly. The robot was an idiot, the kid was an idiot and the plot was really lame, and this comes from someone who thinks most children's shows were awesome (Kyoro-chan, Digimon) or at least acceptable (DICE)

Pokemon
As a kid or as a teen, I couldn't stand more than 10 episodes of that thing.

Sorry that two of them were sequels, it's just that they were horrible shows to very good ones.

Dubs continuously get a bad rap, and I'll admit a lot of them are bad. A lot are also quite good like Hetalia for example.

Yeah, Tsubasa Chronicle: Tori Kago no Kuni no Himegimi was pretty bad but the other two (assuming your referring to Tokyo Revelations and Shunraiki)
I found fairly enjoyable, although I read the manga which covered the ark in between. As stand alones I can imagine they'd be a bit confusing,
but I think they were mainly made for the purpose of giving fans of the manga two essential arcs animated. (the anime could never be completed due to some copyright issues)

Yes, I am referring to them. I'm sorry I didn't recall their names.
As a person who's only seen the anime, the movies' change of pace and intensity came out as terrible. And after I had found out that that happened later on in the manga, I couldn't help but see them as spoilers.
Also, a reason I found the movies bad was that it felt like I watched the whole cast of my favourite shoujo anime slaughtering each other in front of me.
If I would have read the manga first, maybe I would've found the films more enjoyable. However, they're what has kept me from completing the manga.