I think the absolute worst tasting food I have ever eaten is day old mackerel in tomato sauce from a tin that has been left opened in the fridge. It literally tastes worse than cod liver oil and the taste made me gag as a kid when I was forced to eat it. Tastes great with some mayo when it's just been opened, but turns really bad really fast. It doesn't go bad in the literal sense it just tastes awful.
I can't recall any other food making me gag.
In second place is chicken feet. It's not that the taste is so bad it makes you gag or anything like that, the taste on its own isn't actually that terrible. But it's really bland and it's all skin (and not the tasty kind) and fat (or cartilage?) with tiny little fish bone sized bones inside. i can't understand why people in Asia enjoy eating it, it must have been the most bland part of any animal I have ever eaten and the texture was what really put me off.
To put things into perspective, I am a person who will eat almost anything and find it at least mildly enjoyable but I got no enjoyment out of that at all.
I've even eaten Durian and it wasn't bad. Not great, but not bad. It was fried though, I don't think i could get myself to eat it raw especially as the smell would be far stronger.