For me, like someone said it was from Matt's first interview. He just looked the part somehow despite his age he seemed to have a kind of 'wisdom' in his face.
Then when they started bringing out the trailers watching him stride across that courtyard so purposefully I really wanted to see what he was capable of (that was actually the scene in VoV after his first run in with the fish lady and it looked just as good in context)
Think the bits that got me in TEH and proved to me that I had been right to believe he would be great started with 'I don't know yet, I'm still cooking', loved the speech over fish custard when he said 'that must be a really scary crack in your wall': How he went all 'doctory' with 'two points in space and time that should never have touched'
I loved his 'She seems nice your mum' remark about the apple.
In the hospital when Rory says 'he's done it' and he says no and calls the aliens back then says 'Now I've done it' I just loved that line and his delivery of it, then the whole getting clothes scene and up to 'Hello.........I'm the Doctor............basically............run'.
I also loved his expression when he went into the new TARDIS, from that commanding 'I am the Doctor' to the wide eyed wonderment of a child as he looked at what we then couldn't see and said 'oh you sexy thing'
I also loved him 'skipping' (don't know what else to call it) up to the console after he had brought Amy into the TARDIS, like he was still so full of wonderment. then the very touching 'thanks dear' to the TARDIS when he got his new screwdriver.
So, after all that one defining moment?

I don't know, it was almost like he was the doctor before I'd seen him and he's just been comfirming it ever since