Make that favourite per series.
S1: Cyberwoman. I know it's also been called cheesy and over-the-top and whatnot, and, yes, it's a bit cracktastic, metal bikini, pterodactyl and everything, but while Torchwood and I clicked from the start, this was the episode that really got my attention, because it went to unexpected places and then further still, completely unafraid of all the raw emotions it brought up and never even trying to prettify them. And in the end you have Lisa in the pizza girl's body telling Ianto she became human for him, and hadn't he always told her that he didn't love her for what she looked like, and he still can't shoot her, and absolutely nothing is resolved, and... damn.
Also, because without this episode I'd probably never have become interested in either Ianto or his relationship with Jack. (More about that in ten days or so.)
S2: Adam, because it shows a gentler, more vulnerable side of Jack that doesn't come out very often, but that we need to see occasionally for the character to remain likeable. I'm not a huge fan of Exit Wounds, and there are probably a lot of things about the story of Jack's childhood that could have been done better or differently, but at least in this episode I completely believed what it all meant to him. Maybe my favourite Jack episode overall, pre-CoE.
CoE: Day Five, for the sheer Greek tragedy epicness of it.
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