Constitutionally speaking all the roles are of equal weighting, the Prime Minster being a head among equals.
In reality if a politician is seeking a ministerial position it is more likely to be in a field that is engendered with more power, and indeed heading a Ministry is often considered more prestigious than 'Government Department'.
So one potential hierarchy is the PM followed by Chancellor, and then Home & Foriegn Secretaries. This would followed with Defence, and then Health, Education and Transport in relatively equal measure given those departments sizes.
After that, Environment is usually high profile, Trade and Industry too, but Communities, DWP, Food and Rural affairs and Culture, Media and Sport would be considered the officesof lesser significance.
That said, current Home Secretary is alleged to have said, upon being told of his appointment to his previous job at the Department of Health,
"Oh no, not Health!"