You're picking the best of Skrtel and the worst of Agger to compare and it isn't fair at all.
Skrtel looks lively and makes more interceptions and last ditch tackles, sure. But that is because he f**ks up so often that he has to. I'd rather have Agger not lose the ball in a defending position than have Skrtel lose it and have to scramble to recover it.
Penalties? Don't get me started. This won't be the last stupid idiotic shirt pulling nonsense that Skrtel does this season.
Quite frankly Agger gets a lot of stick for marking and while a lot of it is deserved, the rest comes from Skrtel or Toure being out of position or caught in possession. He isn't the best at set pieces. Neither is Skrtel. Neither is Toure. Bit when I see the team sheet I have more confidence in Agger to provide stability than I do in any other defenders bar Sakho.
There is an anti and pro bandwagon for all of our players bar Suarez and Sturridge. The reality is in between and not black and white.
What's Agger good at then? He can't defend set pieces, he can't take a man one on one, his passing is hugely over rated (he infact loses the ball more often than Skrtel in our third, but because he makes forward runs, he comes out smelling of roses), he's not a great organiser, he doesn't cover the left flank when the LB pushes up...what does he do? People constantly praise his comfort on the ball. Shove him into midfield then because I want defenders who can defend and Agger can't.
I probably over rate Skrtel, or come across as doing so - he's not fantastic, but he's solid and effective. People sure make a Messiah of Agger though and I can't see why. Great servant, but not good enough.
Sakho > Skrtel > Agger > Toure for me.
We can improve significantly on three of those.