I think that pretty much all have been said but I'll add one more thing ;D. There's a really powerful scene near the end of 3x13 with Morgana and I totally LOVED it. Merlin and Arthur together with Leon, Lancelot, Gwaine, Elyan (Gwen's brother) and Percival (who are now all knights - as Arthur made them ones) get into Camelot to save Uther and steal the cup of life. They fight with immortal knights while Lancelot and Merlin search for a cup and there's a scene when Merlin almost has it but before he can reach for it, Morgause runs into the chamber and throws him on the wall. (I think) she's about to kill him but suddenly Gaius appears (almost out of nowhere) and uses his magic to throw her on the wall, then Merlin does it too and Morgause lies unconscious on the ground.
Then Morgana runs into the chamber and when she sees her sister like that she starts to cry. And OMG, what happens now is just so powerful, I think it might be my favourite scene this season - Katie did such a fantastic job, I can't even...
she cries over Morgause, who looks pretty much dead and Merlin tells her it's over but Morgana tells him it's just a beginning and screams, at the same time using her magic. It's so amazing because there's so much pain and grief in her screams and her magic bursts out, so everything around them starts to fall and windows break.

Here you have a gif, if you'd like to see a bit of it:
http://i55.tinypic.com/9zsgah.jpg (by banjothequeen)

And sorry for such a long comment, but I love this scene so much that I couldn't explain it in a few words xDD.