Rank our centre backs from most to least essential

One, two, if not three will probably be moved on in the near future. Centre backs can play for a long time, but for the long term security of our squad a bit of youth would be nice. So taking wages, ages, injuries, ability, everything into account, who is the most and least important to keep hold of?

For me, from most sensible to keep to most sensible to move on:
Tomkins
Kelly
Dann
Cahill
Sakho