I have to pay off on a bet…but I want to have fun doing it. I owe someone a case of cola, and they said “I want cola in the worst way!” Always happy to oblige, I want to buy a case of the worst store brand cola out there. No diet-just regular cola, and if you can actually taste the can it comes in that would really make my day. 
I’ve heard Pepsi Peeps* is pretty gagging (but I haven’t been masochistic enough to try it for myself). I imagine it’s seasonal, so stock up while you can.
*“sickly sweet with overtones of marshmallow”
eta: it’s the one with the yellow label and cap
Starlight Coca-Cola got quite a response from the Board. I see it now sitting in the clearance aisle at Aldi.
Just regular cola flavored, please.
Don’t know if it can still be found but the worst cola I remember was called Mello Moon cola. Picked it up at a small store and couldn’t finish one can.
Really? Because I’ve been looking to find a can or bottle of it and haven’t seen any (and I’ve checked several Aldis). I have a friend who didn’t try it when it was around and wants to.
Getting back to the OP, keep an eye out for Moxie. I like it but it’s what’s generally called an acquired taste. Although it’s mellower than it used to be; Coca-Cola lightened up the flavor when they bought the company in 2018.
I tried coffee flavored Coke once, at an airport. It was vile.
Vanilla anything is an abomination. And it’ll be the worst fake vanilla corporate mega-bucks can buy.
Try that.
Assuming cheap=worst…
https://www.dollartree.com/stars-stripes-cola-275-liter-bottles/219175
ETA- A review from that site: “i had some psychopath thing and when i drank this i woke up in a dumster and im pretty sure it cured something i had”
Again, just straight cola flavored cola.
Moxie isn’t really a cola. It’s more towards a more bitter version of root beer (with gentian root being a key ingredient that contributes bitterness). I have no idea what it was like before, but I quite liked the stuff when I ordered it online in a fit of COVID boredom. I was actually disappointed it wasn’t more bitter, because I could barely taste the gentian in it. That said, I love extremely bitter things, so my taste buds may be warped.
As for the OP, I’ve never had a store brand cola or off-brand cola that wasn’t drinkable. They’ve all been fine, as far as my tastes are concerned.
Really, your best bet is to scrounge about “dollar” stores and find a case that’s been sitting on the shelf for too long.
Both pepsi peeps and starlight coke are absolutely delicious, the worst soda is diet coke.
No diet cola, as pointed out in the OP.
I try one of those grocery stores that stock foreign foods and buy a case of cola from some place like Ghana or Laos or Uzbekistan.
For values of disgusting, and sticking with straight colas, maybe Nitro Pepsi?
I actually like the vanilla nitro Pepsi option, where the creaminess plays into the flavor, but the ‘normal’ option tastes like oversweet, under-carbonated Pepsi, with an unusual mouthfeel.
Plus, if they don’t open the can properly, you can have them make a complete mess of themselves without having to shake the can or anything.
Since it comes as a 4 pack though, and is substantially more expensive, I probably would avoid it for paying off a bet for a case.
If I had to pick a ‘normal’ but (to me) unpleasant soda, I’d probably pick RC which always tasted off to me. But other people prefer it. And I’ve never been cheap enough to buy store brand colas to evaluate. At a guess, whatever Walmart sells, because they always seem to cut the most corners.
I feel like I am going crazy, nitro pepsi is SO good. If they had a zero sugar version I’d drink nothing else ever.
Personal preferences differ obviously but I like Sam’s Club cola.
As I said, I like the Vanilla option, but for someone drinking it blind, it could be a shock. Like I said, for values of disgusting. 