Over the years, there have been many alternative realities on this show and it's one of my favorite concepts on Smallville. Alternative realities were included in the following episodes: Luthor, Reckoning, Infamous, Kent, Pandora...just to name a few. My favorite alternative realities are:
Earth-2, which appeared in Luthor and Kent
I also liked what happened in Reckoning and Infamous, which I feel are similiar episodes to each other.