The "Top 3 Episodes Of Nu-Who" meme from The Medium Is Not Enough looks to have started a trend! Now Toby at Inner Toob wants to know the "Best 3 Guest Stars Of Nu-Who". I personally take that to mean actors who only appeared in one story...
So, here are my choices -- feel free to add your own:
It's a cliché to adore Carey Mulligan's Sally Sparrow, but I think the widespread adoration is justified -- because her character is the only nu-Who guest star to successfully carry a whole episode. Marc Warren tried valiantly in "Love & Monsters", but his drippy character was forgettable. The script for "Blink" is genius, but if audiences didn't respond to Mulligan's performance it wouldn't have worked half as well. But over the hour, Mulligan ensured Sally stood out as a believable, intelligent and beautiful young woman, who's resourceful, relatable and determined. I still hope she'll find her way inside the TARDIS some day.
I don't think Jacobi would have worked as The Master in the climactic episodes that followed his transformation into John Simm. It's clear Russell T. Davies wanted an "evil twin" of David Tennant's Doctor, and Jacobi's gentleman villain wouldn't have worked in that context. But Jacobi remains compelling as the benevolent Professor, before he reverts to his true Time Lord identity in nu-Who's most spine-tingling sequence. You can see the transformation take place deep inside Jacobi's cold eyes. And, for the few minutes he appeared before being shot "dead" by his assistant, Jacobi's Master was evil personified.
Callow can play upper-class gents in his sleep, so nu-Who's version of Charles Dickens wasn't a stretch for the noted thespian. Regardless, it's easy to imagine Dickens would closely resemble this fun interpretation. Callow's a big fan of the Victorian author, so he obviously brought a lot of insight and knowledge to bare on Mark Gatiss' script. The result is nu-Who's best historical characterisation, and one that blew off Dickens' cobwebs to make him cool for school.
Honourary mentions: Julian Bleach as Davros ("Journey's End"), Christopher Ryan as General Staal ("The Sontaran Stratagem"), Sophia Myles as Reinette ("The Girl In The Fireplace"), Harry Lloyd as Jeremy Baines ("Human Nature"), Lesley Sharpe as Skye Silvestri ("Midnight"), John Simm ("The Sound Of Drums") & Dean Lennnox Kelly as William Shakespeare ("The Shakespeare Code")
