The FMA movie - Conquerors Of Shambala. I was coming off the 03 series (I don't think Brotherhood was a thing back when I watched them), which I kinda liked and I was really intrigued by the premise. I remember HATING it back then. Haven't rewatched it, but might be my biggest anime disappointment ever*.
Oreimo - I actually liked the first episode and like half of the second one. Thought I might as well give it a chance. And it nosedived. I actually played solitaire while watching to not get bored. In the end I only liked the first episode and like, the last two or so (plus the first two or three specials). Skipped the second season because I didn't think it was worth my time, and it might have been for the better.
Submarine 707R - Most of you haven't heard about it and I'm not surprised. I only have this because someone was selling some anime DVDs and I bought it all.
Anyway, it's a submarine anime. The fights might be the best thing in it, but they're still half decent at best. It's boring. It's ugly.
It doesn't even have a coherent artstyle. It has one of the most out of place and useless loli scenes. It even has no characters -
see that cover? No one except for the captain matters or IIRC even appears for much time outside of being in the background at all.
Obviously, it's an adaptation (more like a promo, since it's just 2 OVAs) of a manga, so it might have been fleshed out later, if it was to be continued. No idea about the source material, but it wasn't and as a standalone thing it's just not good.
If I didn't drop it hard, Kampfer would be my another choice. I actually checked out the manga for a bit and while it wasn't great, it was OKish. Plus I kinda like some of the designs.
*aside from the second or third season of Minami-Ke.