Easily "a million little fibers" and "the return of chef". this season and a bit of last season is when they start relying more and more on shock value and gross-out humor, and when it's not paired with something intelligent to say, it doesn't really work for me.
i also didn't like "tsst" that much just because it's kinda hard to believe that cartman would go down that easily. it seemed out of character that he didn't set some convoluted trap for cesar. but the first half of the episode was still good.
"smug alert" was also a dud for me. basically, the argument of the episode (smug is like smog) was just not very well put together and mostly came across as cringe.