Doctor Who: Every Modern Companion's DEFINITIVE Episode

Jack is a slightly awkward case in a list like this because his defining traits are spread across two shows. In Doctor Who, Jack is a secondary character, but in Torchwood, he's the lead, and much of his character development happens in the latter (if I'm allowed quintessential picks from Torchwood, I'm going with 'Children of Earth' or 'Captain Jack Harkness'). However, we're talking about the main show today, and when it comes to his appearances there, I'm going to bat for Utopia. Yes, the Empty Child is a fantastic introduction to Jack and a pretty good case study of what makes him tick, but the Jack in that story is not the fully-formed Jack we come to know. 

Utopia is our first introduction to the version of Jack that has been left behind by the Doctor and cursed with immortality. He's lived lifetimes, not all of them pleasant, and he's got a season of Torchwood under his belt already. At face value, not a lot seems to have changed since his time with Nine, but, as 'Utopia' unfolds, we see a version of Jack who's carrying a lot more depth than last we saw him. He wears the Captain Jack persona as a mask, and there are times in this story where we see that slip. What Utopia does particularly well is bridge Doctor Who Jack and Torchwood Jack. He's still fun and camp and flirty, but the guy is also carrying a metric tonne of trauma and some pretty gnarly abandonment issues. 

The scenes between the Doctor and Jack are something special in this story, particularly the conversation in the radioactive chamber. I really like that there's no grand monologue, and that the story just slows down and lets these two characters unpack their baggage. It immediately redefines their relationship with each other, and we see a version of Jack who is now balancing his admiration of the Doctor with a dash of resentment. We also get to see Jack's immortality be useful to the plot on multiple occasions here, providing main-show only audiences with a solid introduction to his new 'abilities', and probably the best example of the main show making use of them, rather than turning them into a gimmick or ignoring them entirely as later stories would go on to do. 

In summary, Utopia is Jack's quintessential story because it introduces audiences to the version of Jack Harkness we're now familiar with, whilst still allowing him to be the charming, swashbuckling joker he was in series one.