Two things are prominent in my memory for being utterly ghastly.
The first I called "worm stuff." It's what our parents gave me as a kid when they suspected some intestinal wormage. I don't recall the product name but it was a powder that came in satchets. The powder was disolved in tap water to produce a purple solution that had a uniquely vile taste and smell that filled your mouth, nose, ears and brain. It had my gag reflex going like a ungoverned steam piston *does a little sick in mouth and swallows*
The second, much more recently ingested was a radiographic contrast agent called Gastrografin. I was started on this about 16 hours before an abodominal CT scan and had to drink glass after glass. Absolutely f-o-u-l. There was a hint of aniseed but that failed to make it remotely palatable. Good lord - my uvula is twitching just thinking about glugging the stuff down.
Oh and Robinson's bitter....