i'm also trying to figure that out. the imdb entry for the show doesn't have any details for the cast yet. Spoiler i thought it was the same actress who played eye patch lady / madame kovarian from the demon run / a good man goes to war series, frances barber, but there's nothing on her imdb page or anywhere else about her having a new bit on dr who or anything of that nature. i liked the new episode. tom baker was my first doctor, and david tennant was the most i've liked any doctor since. but matt smith is a respectable third. and i hated him at first. i hate capaldi less than i hated smith when he first started in the role - so i'm optimistic.
same, for the most part. personally, i find it almost impossible to rank them, as they each are so different in what they bring to the role, and the role morphs (regenerates?) appropriately. it's not like james bond, for example, where the actors are filling in to play the same character, these really are different characters with only the name and the tardis in common. part of the brilliance of the writing is this aspect of the character, if you ask me, because it really does give so much more freedom to both the actor and the role he's playing. looking forward to the rest of the season.