The worst job I had was no job.
Apart from that it must've been before I went studying back in 2001, when I worked in a Styrofoam factory. I did different things there, though, so it wasn't all bad (loading lorries was OK, as was collection and disposal of broken stuff and the collection and storing of finished produce) - what really was bad was working at the machines, several at a time - getting the newly produced pieces out, putting them on a palette, wrapping them up. It was boring and stressful at the same time. Boring because it was, well, boring. Nothing really happened. Stressful because each worker had three machines to look after. Some of the pieces produced were quite large, so they piled up fast and would jam the machine, which would take no heed of this and just crunch the piece it produced before, which caused it not to close tightly, which in turn resulted in a piece of rubbish being produced (+ the crushed piece that was also now fit for disposal).
Add to that the warmth and humidity (Styrofoam is produced by pouring gas filled pellets of styrene (size varies, but it's a bit like sand) into moulds, which are then treated with hot steam. The heat makes the styrene elastic and adhexive, the gas inside expands and the formerly tiny pellets become larger pellets and stick to each other.) It was summer when I worked there, so the temperatures were already quite high. One week I was lucky and got a night-shift, but day-shift was quite ugly.