The best CB pair was Smalling and Jones, by far. Actually their statistics are quite similar, according to squawka:
Smalling (in 25 games)
Clearances-163, Interceptions-58, Blocks-16/ Avg. defensive actions: 9
Duels Won (57%)
Tackle-23/52(44%), Take on-16/19(84%), Headed Duel-67/102(66%), Fouls commited-34, Fouls suffered-11
Avg. passing accuracy-88%
Jones (in 22 games)
Clearances-175, Interceptions-70, Blocks-6/ Avg. defensive actions: 11
Duels won (57%)
Tackle-41/80(49&), Take on-16/20(80%), Headed Duel-46/77(60%), Fouls commited-55, Fouls suffered-29
Avg. passing accuracy-85%
Jones has attempted more tackles while Smalling was involved in more headed duels which can be explained by their different styles of defending. Jones is more aggressive, tries to show more urgency and get in front of the attackers to intercept the balls coming towards them. On the other hand, Smalling is more of a calming presence, the defender we rely on to deal with crosses and all sorts of threats in our box.
They compliment each other rather well and they have stepped up, as Smalling suggested a few days ago. Their stats are actually good but there is still room for improvement. Smalling has to become even better at aerial duels and Jones must learn to control himself and not give away so many fouls in our half of the pitch. For instance, Mertesacker's percentage in aerial duels this season was 70% (108/154), he committed only 21 fouls and his avg. duels won percentage was 65%.
Jones did better than ok in most cases and Smalling was the best CB this season but not by a mile. But as they say in my part of the world, you would suffer less if you lost your eye than if you lost your good reputation.
The other CBs don't even come close to those two, you can check the stats if you want. As for the FBs, i don't believe we can draw safe conclusions because in modern football their role is more complex than it used to be. Valencia was quite solid defensively (although he still has problems when he has to defend in the box), he was very important in our build up play (i believe he was the 3rd player with the most touches per game) but he offered very little in the final third. We had the same issue on both wings and it was on of the reasons we couldn't open up tight defenses.