I can't say I've seen too much completely terrible anime simply because I don't watch a whole lot and what I do is generally well-liked, but here are two I found really terrible:
Arcade Gamer Fubuki [http://tvtropes.org/pmwiki/pmwiki.php/Manga/ArcadeGamerFubuki]: This one doesn't even have a Wikipedia article, so you know it's quality. The premise is that Fubuki is a standard shy, naive teenage girl that calls upon ungodly video game skills through a magical pair of panties. When she does this, she levitates upside down and flashes everybody. It manages to be simultaneously lowbrow, unimaginative, obnoxious, and just plain unfunny.
Puni Puni Poemy [http://en.wikipedia.org/wiki/Puni_Puni_Poemy]: I know this is a spinoff of Excel Saga and therefore has Shinichi Watanabe's cult following, but I found it headache inducing. It's one of those bizarre, over the top things similar to Panty and Stocking with Garterbelt and Invader Zim (both of which I like better) where if you don't fit the vague mental state it's targeted towards, it's just excruciating. I will give it credit that out of all the dozens of jokes per minute I didn't find funny, there's one moment where I couldn't stop laughing for a few minutes (forgive the dub).
Arcade Gamer Fubuki [http://tvtropes.org/pmwiki/pmwiki.php/Manga/ArcadeGamerFubuki]: This one doesn't even have a Wikipedia article, so you know it's quality. The premise is that Fubuki is a standard shy, naive teenage girl that calls upon ungodly video game skills through a magical pair of panties. When she does this, she levitates upside down and flashes everybody. It manages to be simultaneously lowbrow, unimaginative, obnoxious, and just plain unfunny.
Puni Puni Poemy [http://en.wikipedia.org/wiki/Puni_Puni_Poemy]: I know this is a spinoff of Excel Saga and therefore has Shinichi Watanabe's cult following, but I found it headache inducing. It's one of those bizarre, over the top things similar to Panty and Stocking with Garterbelt and Invader Zim (both of which I like better) where if you don't fit the vague mental state it's targeted towards, it's just excruciating. I will give it credit that out of all the dozens of jokes per minute I didn't find funny, there's one moment where I couldn't stop laughing for a few minutes (forgive the dub).