From the beginning I would have said John Schneider's portrayal of Jonathan Kent. Every sci-fi show needs to brought down to earth. In Seasons Four and Five, however, the highlight is Erica Durance's Lois Lane. Sure, Lois's presence in Smallville is apochryphal, but she gives the slow-as-molasses melodrama a kick in the butt.
In last night's episode, Clark saves a man in Metropolis who turns out to be a hitman who can turn invisible. Said hitman tries to repay Clark, first with a plasma screen TV, then by choking out Lex Luthor.
Clark tracks the guy by super-hearing his heartbeat. One would think if the guy still has a heartbeat—he doesn't become mist or something—Clark would be able to see him with x-ray vision, but hey. You may argue that Clark wouldn't be able to distinguish the killer's skeleton from everyone else's, but then would he have any better luck distinguishing the killer's heartbeat from everyone else's...?
At the end of the episode, Lois, who was attracted to said hitman, has a heart-to-heart with Martha about her uncanny bad luck with men: "Why can't I just meet a decent guy from a decent family who isn't a psychopath?"
Meanwhile, decent Clark is still bemoaning the fact that Lana is dating Lex. Let Lex have her, for all I care. Lois is more fun.
Trivia: In real life, Erica Durance is married to David Palffy, a.k.a. Stargate SG-1's Anubis.