Choosing between this and season one for the top spot was an extremely difficult task but the chilling performance of John Lithgow and perhaps the most memorable moment in the shows history just tipped it for me. Season four was fantastic for many reasons but the one thing it is most memorable for is its antagonist, Arthur Mitchell aka, The Trinity Killer. Admired by Dexter at first, Mitchell lived a seemingly ordinary and at times charitable life but as his story unravelled it was clear all wasnt well for the seasoned serial killer. Lithgows performance achieved mass critical acclaim and rightfully so. His chilling portrayal left a chill down many peoples spines and his final act in the show, killing Dexter's wife Rita is perhaps the most shocking in the show's history.