The best replacement for Peter Capaldi would have been Alex Kingston, who was so good she stole every scene that she appeared in. one in particular stands out.
'The Wedding of River Song' alongside Matt Smith, Karen Gillan and Arthur Darvill. season 6 episode 13.
"Having finally accepted his fate the Doctor travels to Lake Silencio for his final day. But one woman refuses to let time take its course."
the episode was a masterpiece by Steven Moffat whose standards Chris Chibnall falls so far short, the deficit is embarrasing.
you only have to watch Alex deliver the following lines:

"River Song: I can't let you die...
The Doctor: But I have to die.
River Song: Shut up! I can't let you die, without knowing you are loved. By so many, and so much. And by no one more than me."
why Chibnall did not give Kingston a gig next to Whittaker? because it would have shown the chasm in abilities. and it would have been obvious to all that Alex Kingston was a natural and powerful doctor who.