Keane and Holgate are our best pairing for me at the moment. They have both really improved over the past year or so, and have cut out mistakes in their game.
Mina is yet to do that, as he hasn't really had a solid run in the team and so has very little form. No doubt that Mina has everything needed to become an absolute beast at the back - height, skill, composure, heading. He does seem to lack pace at times, but this can be compensated by his reading of the game and positioning, which should improve.
This might not be a popular opinion, but Keane is our mainstay at the back. Consistent performer, and won't necessarily look as classy on the ball, but he is a manager's dream in that he does the basics very well, and puts his body on the line.