For me, it's a toss up between Illusions of Grandeur, Never on Sunday, and Smart Kids.
Smart Kids- where was the plot? Seriously, just where? It kinda meanders along and doesn't really hit its target. And the annoying thing is, it could have been a really great introspective episode for Clark- but the writers didn't dig any deeper.
Illusions of Grandeur and Never on Sunday- the whole black magic/illusion thing just rings really hollow to me, and I dislike the idea that Clark would ruin someone's life and just *not care*. That's totally out of character for Clark.