Smalling and Bailly for me.
Smalling is good in the air, and probably has the best sense of positioning out of our current defenders. Bailly is okay as an aggressive, front-foot centreback. While he clearly struggles with the team's more reactive style of defending, the same is true of the other candidates, and he might be able to get out of trouble once or twice more than the rest because of his pace, agility and (this is obviously very subjective) tackling accuracy.
This pairing won't be very good at playing from the back. Unfortunately, our attack at the moment really needs someone who can play accurate and incisive early passes. Lindelof has shown this ability elsewhere, but not yet at United, which is hardly a surprise: being a defensive liability does not bode well for one's confidence.
Both Bailly and Lindelof have some improvements to make, and they will need game time to do so. However, I feel that Bailly's shortcomings are more decision-making and lapses-in-concentration, with which game time can help, whereas Lindelof still has some work to do off the pitch. It's a harsh assessment, and I hope I'm wrong and that his time at the World Cup is a more accurate reflection of his current abilities.
I don't think we we need a pairing, we should play 3 CB's Bailly - Smalling - Lindelof, it would help us get the best from the existing squad by relying on wing-backs to give us natural width.
Agree with this. They are probably the most balanced combination given our current options. But Mourinho's reluctance to field wing-backs against back-four formations is a decision that makes sense within his tactical framework, so I doubt we will see it.