When I think about the worst anime I watched, having just shit animation or rushed pacing doesn't really add to the suckage since it's like arguing that a child with no limbs is the worst runner ever.
For a truly awful anime it needs to have the ability to be genuinely good. And in that regard the worst anime I've seen is Re

reators.

It starts pretty solid and becomes a Oscar level self fellating affair by the end. It has the budget, length and setup to be great but decides that rather than sticking to the characters it needs to talk on how being a mangaka is the most awesome thing in the world.