Hm, I guess this is tough. There are a lot of anime that I sat through entirely, but I finish them because I was wondering what the heck was so popular about them (Mekakucity Actors, Ao Haru Ride), so I can believe that I watched all of those haha. But anime that I can't believe I watched through? Ano Natsu de Matteru, Soul Eater NOT, Galilei Donna, Gingitsune, Ryuugajou Nanana, and Inou Battle wa Nichijou-kei no Naka de (don't know the English name, I just have it up on MAL in Japanese sorry) are probably a few. Alright that's more than I expected :P
Ano Natsu de Matteru - I don't remember all of the specificities but I really despised this anime after watching it. I thought the alien aspect of the show was really poorly handled and the romance was pretty awful and melodramatic for no reason. I won't comment on anything else because I blocked it out from my memory but I hated it so much I don't understand why I completed it.
Soul Eater NOT - How can the spin off to such a great show fail so horribly? I probably watched this just to have an excuse to listen to the opening song every week. But honestly, maybe it's because I expected it to get better. But it was pretty clear early on that this was going to be bad. The characters were cookie-cutter stereotypes, problems were never resolved, and the anime couldn't pick which genre it was going to be, which caused it to fall short in all aspects.
Galilei Donna - I honestly don't even remember what this anime was about and I don't remember it being spectacular. Which means that I probably should not have sat through all of it. I don't think it was terrible I just think it was lacking something (besides from a great opening song that I still like)
Gingitsune - Another bland show that, if I had watched now, I would have dropped halfway through. The characters were alright, there was just no resolution. Honestly I don't even remember the ending.
Ryuugajou Nanana - When I heard about the concept, the show seemed a lot cooler than it actually was. Too many sideplots and I'm not sure if this was supposed to be a comedy show or not (so, awkward comedy and not that great delivery)
Inou Battle - I know this was supposed to be a slice of life show but it was kind of really bad? The storyline with Andou and the red-haired girl was alright and I didn't hate that part, but it seemed that they were planning on putting an explanation for their superpowers but they never went through with it? And one girl was kidnapped one episode and everyone just forgot about it... what? If you took away the superpowers you would have a show that was almost exactly the same, and I get that it's the point (that their ordinary lives kept going even though they have superpowers), but that gave me the impression that they were only using the excuse of superpowers to sell a mediocre story.