To truly judge each of the modifiers, you have to think of the way they work with each of the weapon classes individually.
Obviously only polearms get the stagger, and honestly it's the best modifier for them. A close second being bonus against shields, because the weapons that use it are generally quite good. Knockdown eliminates the chance of polestagger, so it's far less useful considering it gives nearly the same result but happens far less often. There's only one crushthrough polearm, so whether this is good depends on how you feel about the long maul. I am not a fan.
For 2handers, their crushthrough weapons can be simply unstoppable in certain circumstances. Their knockdown weapons, while not very popular, are actually quite decent (some of which also have crushthrough!). Their bonus against shield weapons are mostly bad, with few exceptions, and all of them (save the fighting axe's alternate mode) are unbalanced.
1handers have some very good bonus against shield weapons, and a handful of moderately decent knockdown weapons. I'd say it's a tie for them, with bonus against shields possibly edging knockdown out by a smidge.