Can't recall anything worse than Tokko so it must have been awful as hell.
The anime I hate the most tend to be longer, if they are short I don't get the time to really hate them. That's why I rated Mars of Destruction higher than Black Butler, even though the former probably is far worse. But to be fair, I probably gave MoD a too high grade, I just didn't want to waste the low grades in case something even worse came along.
Voltage Fighter Gowcaizer is pretty bad, long enough to be boring, and looks bad from the beginning.
I don't really know how to answer this question, of course there are the anime generally regarded as trash, but it would perhaps be more interesting if I said something popular? Or at least that are not always on the lists of top ten worst anime ever? If that's what you want, perhaps
idk I try to avoid the worst anime and these are also regarded as terrible, and anyway I always watched Dragon Crisis before Kore wa Zombie when they aired because Dragon Crisis had a better OP.
If this was what you really wanted:
Neon Genesis Evangelion
This gave me cancer
Fetish incest otaku pandering trash
r-15, but I loved it still...
I mean, it's not the worst anime that I know of but it's the worst one that I've watched. Dropped after the first episode. Couldn't bother to give a shit or remember anything that happened, if anything did.