Well for me, it depends on what kind of captain you want.
Do you want something like Kirk-style, Picard-style or something in between?
If you wanted something towards the Picard-style of capt'n'ing, then I'd go with Laurence Fishburne as mentioned before,
or perhaps even William Petersen,
not because both of them were on CSI Las Vegas, but because I like both of their acting styles and would play a decent starfleet captain that has a good head on their shoulders.
If you wanted to lean towards the Kirk-style, I'd go with either Colin Farrell
or Bruce Campbell.
I know I know, a lot of people don't care much for Colin F. these days, but I was a fan of his acting before he became main stream when he was in Tigerland and he can be a really good actor when he wants to be, lol.