I can say mine was in my teenage days when I had no money so I begged beverages off my equally bereft peers. I made a rocket-fuel mix of beer, wine, vodka, schnapps mix that worked ok until I added Baileys, since the concoction congealed with that ingredient. I still drank it. So what’s the worst alcoholic beverage you’ve ever imbibed?
A strawberry wheat ale I brewed when I first started homebrewing; the first one to get fermented in a plastic bucket rather than a carboy. It tasted like… a plastic bucket.
Bong water. Wait, you did say alcoholic? OK then, once I was preparing for a camping trip…had two partial bottles, one of everclear and one of Canadian Mist. For convenience, I just dumped them both into a large unbreakable plastic jug and took it along for a campfire tipple. It did,in fact prove to be good for starting campfires.
On my first (and only) trip to England, I wanted to sample the regional drinks and foods. Cornish pasties were shite btw.
But Somerset promised great things with cider. We called into a farmhouse that advertised itself as having the best in the district, bought a couple of bottles, then retreated to our modest B & B for the evening.
That cider was…well…undrinkable, and I would say it’s the worst liquid I’ve every voluntarily swallowed.
Gin and kirsch mixed with orange Kool-Aid.
I had some buddies that worked the floor in a small Mexican restaurant. They would combine all the unfinished drinks into quart mason jars.
Or:
Apple juice, sugar, and baking yeast, in a 2 gallon plastic apple juice container. Put a balloon on the top of the container and vent it every day. Two weeks later, get drunk, a headache, and diarrhea.
My favourite largish microbrewery (Granville Island Brewery in Vancouver, BC) puts out a “Winter Ale” every year. The first time I tried it, I expected a festbock, which it isn’t. I don’t think my expectations had much to do with my perception, though - because even after going back to it knowing that it wasn’t a festbock, it tasted exactly the same: Like a bucket of water which had dirty socks steeping in it, which somebody had tried to freshen up by adding artificial vanilla extract. Blech. I sometimes end up with this in “sampler” cases, and it’s the only beer that consistently ends up going down the sink. I am not a fussy beer drinker but it is vile.
However, if we aren’t limiting ourselves to commercially available products, back in the 1980s: I tasted a home-made liqueur which my father had made - alleged to be a Drambuie analogue. Sometimes when I first wake up in the morning after being ill for a week or so, I think I can still taste it. You could approximate it by sweetening kerosene with honey and then giving a tramp an enema with it.
Some grappa. I’ve heard that it all isn’t as bad as the stuff I tried, which was fairly expensive. Worst thing I have ever tasted. Worse than the weird scallop sushi thing, worse than the sea urchin.
Other grappa may be better but I will never find out.
When I was about 14 I shoplifted a pint of MD20/20. It tastes of sweet and rot. 30 years later I’ve never tasted anything that comes close to that vileness.
Cave Creek Chili Beer. Against my better judgement I bought a single bottle. It smelled like a pretty boring American lager. I took a medium sized swig and coughed it up through my nose and then projectile vomited. It had been an idyllic romantic weekend in wine country until that moment.
I’ve brought this up here before, but freshman year college, new student week: gin and cream soda.
Should’ve used rapid-rise yeast, like I did–mine turned out okay. A bit cloudy, and for a time I thought I should have used a weather balloon for the lock, but not bad.
As for the worst drink I’ve had…Green Mamba. I see that there’s something available by that name now–a mix of cider, lager, and fruit juice, which sounds vile enough–but not the stuff I encountered. It was a beer imported from some location in Africa, Hell, or both.
Now, I’m not a fan of beer in the first place. I’ve been heard to call it rotten barley, among less kind things. This stuff actually tasted like rotten barley, though…or rather, like the taste I imagine might be derived from rotten, moldy barley that someone scavenged off a barn floor after chasing the cattle out. It wasn’t just me, either; the dedicated beer drinkers who had talked me into trying a bunch of different beers with them gagged and spat and refused to take another sip. We decided that the stuff was named after a highly venomous critter for a reason.
The Nachos Margarita. Just… don’t.
Gasoline. Trust me on this, never even taste it. Just don’t.
In terms of alcoholic drinks, it would be the moonshine that a friend’s uncle distilled. It was so strong that it tasted of alcohol, and nothing but. I might as well have been drinking a bottle of rubbing alcohol that was purchased from the drugstore. I wanted to cut it with some Coke or maybe orange juice, but I was told not to, as that would insult Uncle. So, on the next round, I just had a bottle of beer.
On the plus side, I can still see! 
Recently, a friend of mine tried to make an alcoholic bubble tea with I think yoghurt, vodka, and I believe blue curacao thickened with alginate for the bubbles. Now I never drank any regular bubble tea, so I’m not sure the consistency is supposed to be this (hmm, I’m not sure there’s a word for this; kind of what you’d be left with if you subtracted the solid part from mushrooms)…eldritch-amorphously blobbish?, but I’m certain this particular dairy-cum-alcohol taste was never meant to be experienced by human tongues…
Southern Comfort. A friend of mine calls this stuff “good whiskey.”
Drinking from the Grog.
I didn’t drink it myself, but one time we were out celebrating a friend’s birthday. The bartender (who knew us) recommended we order him a “Jersey Turnpike” shot, so we did.
He then poured the contents of the little tray where he mixes the drink into a shot glass.
Not me personally, but a friend at university tried to ‘make’ jagermeister by mixing cough syrup and vodka. The resulting faces were priceless but he finished the pint!
Miller Lite