Smallville ran for a whole decade on television, brilliantly expanding on the story of a pre-Superman Clark Kent (Tom Welling). But while the show itself was quite popular throughout its 10-season run, not every season is widely regarded. Season 6 is one that's often considered a declining point in the series, but despite that, it really does have some stellar episodes. In the wake of all the Clark-Lex-Lana drama, one of the most memorable hours (perhaps of the series) is "Nemesis," which tosses Clark and Lex (Michael Rosenbaum) into a series of LuthorCorp tunnels below Smallville, trapping them there until they can find a way out. It's a great episode that challenges our perspective on their friendship, and foreshadows their future.