Ok, so it is really hard to narrow it down to one quote because there are so many fab quotes from the show, so I have a bit of a scene here...it was this scene here that sealed the deal with David Tennant playing the 10th Doctor. And this was the Christmas special...not even the start of Season 2.
Harriet Jones: Who is this!?
Rose: That's the Doctor.
Harriet Jones: Not my Doctor! Or is it just an honorary title that gets passed down?
The Doctor: No, I'm him; I'm literally him. Same man, new face. Well, new everything.
Harriet Jones: That can't be!
The Doctor: Harriet Jones. We were trapped in Downing Street, and the thing that scared you most wasn't the aliens; it wasn't the war; it was the thought of your mother being alone.
Harriet Jones: [realising] Oh my God!
The Doctor: [smiling] Did you win the election?
Harriet Jones: [nodding] Landslide majority.
Sycorax Leader: [angrily] If I may interrupt...!
The Doctor: Yes, sorry; hello there, big fella!
Sycorax Leader: Who exactly are you?
The Doctor: [Grinning] Well, that's the question!
Sycorax Leader: [Bellowing] I demand to know who you are!
The Doctor: [Mimicking] I don't knooooow! [normal] See, there's the thing. I'm the Doctor, but beyond that.... I-I just don't know, I literally do not know who I am. It's all untested. Am I funny? Am I sarcastic? Sexy? [winks at Rose] Right old misery? Life and soul? Right-handed? Left-handed? A gambler? A fighter? A coward? A traitor? A liar? A nervous wreck? I mean, judging by the evidence I've certainly got a gob! And how am I gonna react when I see this? A great big threatening button. Ah-hah.
[sees the control matrix and runs up to it]
The Doctor: A great big threatening button which must not be pressed under any circumstances, am I right? Let me guess, it's some sort of control matrix, hmm? Hold on, what's feeding it? And what have we got here? Blood? :[tastes] Yep, definitely, blood, human blood, A-positive, with just a dash of iron. Haagh. But that means.... blood control; blood control!! Awwww, I haven't seen blood control for years! You're controlling all the A-positives. Which leaves us with a great, big, stinking problem. 'Cause I really don't know who I am. I don't know when to stop. So if I see a great, big, threatening button -which should never, ever, ever be pressed- then I just wanna do this!
[Presses the button]
( the youtube clip with the scene above in it for those interested in the context )
Harriet Jones: Who is this!?
Rose: That's the Doctor.
Harriet Jones: Not my Doctor! Or is it just an honorary title that gets passed down?
The Doctor: No, I'm him; I'm literally him. Same man, new face. Well, new everything.
Harriet Jones: That can't be!
The Doctor: Harriet Jones. We were trapped in Downing Street, and the thing that scared you most wasn't the aliens; it wasn't the war; it was the thought of your mother being alone.
Harriet Jones: [realising] Oh my God!
The Doctor: [smiling] Did you win the election?
Harriet Jones: [nodding] Landslide majority.
Sycorax Leader: [angrily] If I may interrupt...!
The Doctor: Yes, sorry; hello there, big fella!
Sycorax Leader: Who exactly are you?
The Doctor: [Grinning] Well, that's the question!
Sycorax Leader: [Bellowing] I demand to know who you are!
The Doctor: [Mimicking] I don't knooooow! [normal] See, there's the thing. I'm the Doctor, but beyond that.... I-I just don't know, I literally do not know who I am. It's all untested. Am I funny? Am I sarcastic? Sexy? [winks at Rose] Right old misery? Life and soul? Right-handed? Left-handed? A gambler? A fighter? A coward? A traitor? A liar? A nervous wreck? I mean, judging by the evidence I've certainly got a gob! And how am I gonna react when I see this? A great big threatening button. Ah-hah.
[sees the control matrix and runs up to it]
The Doctor: A great big threatening button which must not be pressed under any circumstances, am I right? Let me guess, it's some sort of control matrix, hmm? Hold on, what's feeding it? And what have we got here? Blood? :[tastes] Yep, definitely, blood, human blood, A-positive, with just a dash of iron. Haagh. But that means.... blood control; blood control!! Awwww, I haven't seen blood control for years! You're controlling all the A-positives. Which leaves us with a great, big, stinking problem. 'Cause I really don't know who I am. I don't know when to stop. So if I see a great, big, threatening button -which should never, ever, ever be pressed- then I just wanna do this!
[Presses the button]
( the youtube clip with the scene above in it for those interested in the context )