I would have to go with United.They have a proven defence who have shown over a long period of time they have what it takes, I would consider Rio and Vidic one of it not the best CB pairing in the world currently, Evra is a beast on the left.Ok O'Shea, Fabio or whoever else they play on the right aint the best but they have 2 solid CB's and Evra can defend and get forward.Evra can provide, Vidic can score a few too.
Liverpool have a good defence but lack the ariel ability of a Vidic, I think Johnson will be a class act at Liverpool but his defencive skills have yet to be put to the test, You can see going forward hes great but only time will tell how he defends.Carragher is my favorite defender in the premiership i love his attitude but Liverpool leak more goals than United do.
Chelsea shouldnt be left out, They have a decent backline too even though sometimes they do switch off.They have 2 attack minded fullbacks and 1 solid CB in Terry so not a bad defence by any standards.They are up there with United but seeing as its Liverpool or United i had to go for United as Insua and Johnson are new to the Liverpool backline and i have seen to little of them in a Liverpool to judge how good they are.
Arsenal shipped 37 goals last season, Liverpool shipped 27, United and Chelsea on 24 goals.3 of those defenders from the Arsenal team that shipped 37 goals last season are still their and Vermaelen is a new unproven defender, How can anyone claim that defence is better that United or Chelsea, Last season proved 3 of them were not on par with them, One new signing with 1 premiership game under his belt does not make you the best defence in the league.
im not saying Arsenals defence is bad or Liverpools but Liverpool have 2 i have'nt seen in their backline before and 3 of Arsenals defenders were not on par with United or Chelsea's defenders last season and the other like Johnson and Insua i have not seen enough of to judge.