Up to Merlin 1x08 (the one with the Druid kid).
I really love how mature and serious Merlin is in the conversation with Gaius, revealing that the kid is hiding in Morgana's chambers. I dunno why, but that's the scene that finally sold me on his character. Sure, he was cute and fey and funny up 'til now, and very likeable, but this is the first time I've seen hints of the man Merlin needs to become, and it just makes it all work for me.
Um, that's kind of a random thing for me to fixate on, isn't it? Yes, Merlin/Arthur is banterrific and adorkable, and Gwen/Morgana is all kinds of pretty, and really I'll take any pairing (or moresome) of those four. But I'm not feeling particularly shippy about this show just yet -- because I only ship characters I really love, and I think I need to see this side of a character before I can love them.
It's...I dunno. It's that moment of growing the fuck up and taking responsibility for your actions, and understanding the consequences. It's Jack Harkness initiating emergency protocol 417, or Ianto giving Gwen the coordinates to Flat Holm. It's Turn Left-verse Donna seeing the truck. It's Vince hanging up on Cameron, and Stuart listing off all the actors who've played Doctor Who in order. It's Martha walking away from the TARDIS. And hundreds of other more understated moments that I can't even think of right now, because they're quiet and serious and mature and perfect, and most good dramas have them in spades, and Merlin finally, finally showed me that moment.
So now I'm sold.
ETA: In related episode news:
1. I am SO looking forward to Morgana really becoming Morgan le Fay (this was a good ep for seeing the darker sides of major characters, wasn't it?)
2. THAT KID IS TOTALLY MORDRED ISN'T HE, HOW DID IT TAKE ME THIS LONG TO FIGURE IT OUT. (I paused it just as the kid was returned to the Druids -- think Arthur's about to ask the kid's name, but I had to get that out as the realization hit. OMG.)
3. (watches last 30 seconds of episode) YES HE FUCKING IS. OMG OMG OMG. This is the first episode that managed to surprise me at all, and it is AWESOME.