The worst tech I've been working on is not related to a programming language, is more about the codebase itself.
One of them was in .net, the guy reinvented the wheel creating a custom mvc framework and a custom entity framework, copying from cakephp models, was disgusting and felt terribly wrong to work with.
Then I moved to an old cms written in php on top of an old version of cakephp, that was a nightmare too. Fat controllers and a disgusting db schema, no coding standards whatsoever. Everything so deeply convoluted and connected that was impossible to change something without breaking something else.
The technology itself is never the worst thing, people who thinks they are the best ninja developers, are the real problem imho, and the code they leave behind speaks for them. Yuck