My two cents as a niner:
Avery Brooks was probably the second-weakest actor in the cast, with only Terry Farrell giving a worse performance. That said, it was in large part because so much of the main cast was absolutely stellar - by far the best cast overall from an actorly standpoint that Trek has seen. Given the ultimate structure of DS9 however it didn't matter that much though that Brooks was a comparably "weak link." Sisko had the smallest proportion of lines/scripted dialogue out of any Trek captain (till Lorca). There were many episodes where Brooks basically just showed up to collect his paycheck and did a three-minute scene in his office about something trivial. They mostly only used Sisko when they needed to, and he could, when needed, hit it out of the park.
Still, I know Tony Todd auditioned for the role of Sisko, and I can't help but wonder what the role would have been like in his capable hands. He had only minor parts in Trek, but he was typically stellar in whatever show he was on. He didn't have the star power of Brooks, but he could have brought a lot to the role.