I recently watched Pupa. It is definitely without a doubt one of the worst anime of all time. Please don't waste your time watching it!
Here are some more bad anime that I completed 1/10 - Accel World, Bleach 2/10 - Scryed 3/10 - Kara no Kyoukai: Epilogue, Genius Party Beyond 4/10 - Kanon (2006), Hanbun no Tsuki ga Noboru Sora, Izumo: Takeki Tsurugi no Senki, FF: The Spirits Within, Loveless, Serial Experiments Lain, Iriya no Sora UFO no Natsu, Initial D 1st/2nd/3rd stage, Phi Brain Kami no Puzzle, Aku no Hana I am too lazy to list 5 and up. But at least do yourself a favour and avoid these like the plague. *stuff I dropped 2/10 - Lucky Star, Death Note 3/10 - Ghost in the Shell SAC Tachikoma na Hibi, Clannad 4/10 - Hayate no Gotoku, Sousei no Aquarion, Fairy Tail, Texhnolyze, Initial D 4th stage
Kyou, Koi wo Hajimemasu - I hate this mangaka's works! I don't even know why I watched this..... probably just to laugh. >.> Same with Honey x Honey Drops or whatever it was called. Hyouka & Clannad After Story bored me. I didn't watched the first season of Clannad either so....
I've watched a lot of bad OVAs from time to time and some series can be painful to watch but I believe that Hime-sama Goyoujin was the worst of them. Honestly I don't know why I even bothered to watched it till the end, the humor was aweful; full overused situations and boring characters. Know I've watched a lot of comedy and parody anime and even the worst of them have a few good moments bout "Hime-sama Goyoujin"... nah nothing, nothing at all.
Venus Versus Virus was boring, had bad animation, had idiotic characters, forgettable soundtrack, really colours... in other words a complete waste of time.
i'm sorry.. but Bleach didn't appeal to me
School Days first and then 11Eyes.. The characters haven't any presence, it's too long. The same for Yosuga no Sora, I lost my time. I haven't play  the Vn but I hope it's better.

Well, Bleach isn't a bad anime ( as the manga also) but not a good. It's always the same things, the same outline .. Anyway, it's watchable/readable.
Free. I can honestly understand why people might enjoy watching it though. It's ridiculous so it can be kinda funny. But come on, you gotta be honest it is really bad. Also Amnesia. And Uta no Prince-sama. Seriously just terrible I regret wasting my time with those two.
Sword Art Online, Ouran Host Club, Bakumatsu Rock, and I also thought Free! was bad. The fanservice and characters were ridiculous to me, I also thought it was funny for that. lol. Also people named alot of other shows I couldn't stand like Clannad and Serial Experiment Lain. I'm glad i'm not the only person that thought Clannad was boring. >.> I don't think I'll ever watch the 2nd part.
Sword Art Online - I barely managed to get pass the first half, but I wasn't able to continue the second half and, of course, the second season.
The animes I don't like are: Vampire Knight, Bleach, Naruto, One Piece, Pokemon, Elfen Lied, Azumanga Daioh, Excel Saga, FLCL, One Piece, Attack on Titan.

These are really bad animes.
This is probably an unpopular opinion, but I really dislike SAO. Kirito is just a bad character, and he ruins it for me.
Bad animes that I completed : Sukitte Ii na yo (even if the manga is fine) and RDG: Red Data Girl. 
Bad animes that I dropped : K Project, SAO, Kurokami and Arata Kangatari.