mushrooms... it's like chewing up rotting mulch, and then the bits left in your mouth get all slimy... *shudders* I have a big problem with slimy textures (I can't even eat yogurt without puking), so mushrooms are at the top of my list for sure. Now I eat them with apples, so it's slightly less unsettling.
DXM cough syrup. There was a time during fall of my junior year where I drank 5 bottles of cough syrup within a week and a half, and that [stupid] experience has left me with an extreme aversion to it. I can't even SMELL it now without gagging, and even just taking the recommended dosage has caused me to vomit.
All the chemically tasting stuff doesn't really get to me too bad... MDMA tastes wretched, yes, but as was mentioned somewhere in here before, it doesn't stick around for too long.
And I loooove the taste of coke, oxy, and adderall.