Lex and Clark have to go to Atlantis to help rescue the city. This starts the day after Gene Therapy ends, and you really need to go read that one first for this to make any sense. (Smallville x Stargate: Atlantis Crossover) Clark Kent/Lex Luthor Rodney McKay/John Sheppard 33,975 words
These 2 stories probably only objectively deserve about 3.3-3.5★, but as a fan of both shows, especially of non-evil Lex, I enjoyed this crazy idea enough to round up.