Worst overall: Heineken, god is that stuff wretched
I kinda like (or at least don't mind) Heineken, as long as it comes from a can.
Or those nifty little minikegs...I bought a couple of those before I started kegging. Really not a terrible beer sans skunking.
SA Triple Bock was almost amusingly bad. I had the 1997, sampled probably in 2004 or so. Not even the worst tasting, but somehow the worst brewed beer I could imagine. Soy sauce flavors even. Best used as a meat marinade!
But the only beer I can remember dumping out in abject disgust, even though I do respect it as probably well brewed, is Hanssens Oude Kriek. I wrote three things down in my tasting journal (yes back then I had a journal, lol!). Acetone, vinegar, and manure.