I have to say David Morse
In the 2nd act, it's like there are 2 plays going on: Jim Norton, Conleth Hill and Sean Mahon -- having their 'happy' Christmas Eve / experience, oblivious to the darker story -- and David Morse in a completely different place, emotionally. (And Ciaran Hinds moving smoothly between both worlds)
Every review that says how much heart the show has -- that's all coming from Morse. Everyone else is having a grand time supplying the laughs, playing with lines that are witty and clever. 'Sharky' provides all the pathos in the entire show, MUCH of it silently (since Sharky has relatively few lines in the 2nd act), and Morse carries that emotional weight all by himself
I think it's a stunning piece of work
(Don't misunderstand -- I think all 5 of them do an absolutely amazing job. There isn't a mis-step in the casting or ANY of the performances. Everyone is EXACTLY right for their part. But if I have to pick one...)
You don't go to the dragon without a present - Mark Rylance