To me, Colin Firth's casting seems too expected, too on-the-nose. I have no doubt he could do Henry Higgins in his sleep.
I'd rather see a less obvious Brit choice, like Hugh Grant. He's made his name off being a lightweight romantic-comedy guy, but he surprised me by showing a depth in ''Florence Foster Jenkins'' that I'd never seen before in his films. ... Other possibilities: Hugh Laurie. Benedict Cumberbatch.