Allot of people here seem to really like 'pinkeye' but to be honest, it doesn't get a very high score from me as far as south park episodes go.
but there's a couple halloween episodes that i barely enjoy at all; the 'tegridy halloween special', and my least favorite; 'sons a witches'. they both have similar problems; randy being randy, a somewhat generic random monster terrorizing the town trope, and trey dumping his uninteresting marital issues into the episode (which he seems to do allot of). the more times i watch the heidi/cartman arc, the more annoying it becomes, and butters' mummy isn't that much better.