I'm not sure if that really counts as a Torchwood moment, and it isn't even a single moment, rather a conjunction of moments, but I'll go with it anyway, because I love it so completely:
The fact that Jack's story already has an ending on Doctor Who.
Because if you really think about it, Utopia/LotTL is terrible for Jack, especially after all the immortality angst he already went through over the last century. And he knows it, too. Standing there, a hundred trillion years in the future, looking down into the abandoned conglomeration, the Doctor telling them that this isn't just night, but an ever-lasting darkness at the very end of the universe, Jack already does realise what that means for him, unless the Doctor can fix him. It's very understandable that he thinks it's fantastic that humanity made it so far and is still surviving, because this way at least he won't be all alone. And if this prospect weren't already terrifying enough, LotTL makes it so much worse: Utopia is just that, a place that doesn't exist, the sky isn't made of diamonds, there are no promises, only darkness and despair, and 'You may be out there somewhere.'
The implications of this aren't really addressed after Utopia, but it's the kind of thing that... lingers. And then, right at the end, we learn that it won't be like that, after all. That Jack's story already has an ending, no matter what might happen to him in the meantime. And it's a good one. It's the heroic ending Jack wanted for so long. Jack, in the end, is at peace. He even has the Doctor there, begging him not to die.
Maybe it was a completely impromptu last minute retcon; I don't care. it's a stroke of genius. It changes the dynamic between Jack and the Doctor, it fixes the lingering uneasiness about Jack's eventual fate and gives his character a new gravitas; it fits just perfectly in every way and intentionally or unintentionally adds layers of meaning and depth to the story. There's something Jack says at the very beginning of TW, in Ghost Machine, 'The problem with seeing the future is you can't just sit and look at it. You got to try and change things. Make it happen differently', and when I'm watching Gridlock now, I keep thinking, he's still trying. Of course he also knows he can't really stop things from happening and probably shouldn't, but wrapped in all this deliberate mysteriousness he still gives Ten the key phrase that might have changed so much for him if only he'd been willing and able to read it differently, instead of insisting that only another Time Lord would make him less alone.
Day 01: Favourite Torchwood Member
Day 02: Least Favourite Torchwood Member
Day 03: Favourite Series
Day 04: Favourite Episode
Day 05: Least Favourite Episode
Day 06: Something You Liked That Most People Didn't
Day 07: Favourite Alien/Villain
Day 08: Favourite Minor Character
Day 09: A Scene That Made You Cry
Day 10: A Scene That Made You Smile
Day 11: A Scene That Made You Angry
Day 12: Favourite Quote
Day 13: Favourite Promo Picture
Day 14: Favourite Couple
Day 15: Favourite Couple Scene
Day 16: Favourite Piece of Music
Day 17: Something You’d Like To Re-write
Day 18: Character You Relate To The Most
Day 19: Favourite Outfit
Day 20: Favourite Gwen Moment
Day 21: Favourite Ianto Moment
Day 22: Favourite Jack Moment
Day 23: Favourite Owen Moment
Day 24: Favourite Toshiko Moment
Day 25: Shag/Cliff/Marry?
Day 26: A Torchwood Geek Moment You’ve Had
Day 27: Favourite Location Or Set
Day 28: Where You Think The Series Should Continue To
Day 29: Best Torchwood on Doctor Who Moment
Day 30: Why You Love Torchwood