I have always wanted Depp to play Dr Strange. I think Depp would be his quirkiness to the character that I feel Dr Strange would need to be a more effective onscreen character. While Liam could definitely play the part well, I think it would just come across way to serious.
My problem with Depp right now is that when you say 'quirky', he suddenly becomes a gay pirate. With a strong enough director, I'd certainly have no problem with Depp (NOT BURTON).
I just Neeson, with the entire action-hero evolution of his career, could play Strange with his eyes closed.