Peter Capaldi was lined up a long time ago.
He was lined up long before anyone knew there was going to be a next doctor. In one of the Doctor Who specials that aired on BBC America it was also revealed that it was decided the role was going to be offered to him even before he came in to audition. The story goes that he came in to audition only because he grew up watching the show & loving it as a kid. He figured they were going younger and younger with the Doctors in the new series so he thought he didn't have a chance.
He even told the producers that and they had to struggle to keep straight faces.
For me, though, my favorite doctor is David Tennant. I didn't really care for him at first, but warmed to him slowly. It wasn't until the Human Nature/Family of Blood two-part episodes that really cemented it for me, though. Tennant brilliantly conveyed that the Doctor, while usually quite eccentric and carefree, could also be as cold and deliberately cruel as needed in order to stop a threat. Those episodes really added a new layer and dimension to the Doctor for me. More accurately, the last few minutes of the second episode did.
It showed that if you really get him mad, he will finish you and not in a merciful way.