It's just that simple.
Think about:
-Overall Utility to it's faction
-Age
-Quality
-Material
-Cost
If you want, make a list of the one mini per faction that needs updating the most:
Then, which is the worst of the bunch.
Space Marines:
Space Wolves:
Blood Angels:
Dark Angels:
Grey Knights:
Chaos Marines:
Chaos Daemons:
Tyranids:
Necrons:
Eldar:
Dark Eldar:
Orks:
Imperial Guard:
Tau:
Adepta Sororita:
Go!