Which bane do you think is the most painful/punishing to get applied to you? Speaking purely of the bane itself not other uses (as that can shift very suddenly)
Blight: Remove a die. Depending on the unit comp this could be brutal (1 die instead of 2 on a Threat4-5 spearstar...ouch) or mostly annoying (still cuts damage but a small unit losing a die points-wise isnt as painful as a spearstar losing damage)
Stun: Cancel bonus dial. If the target can easily get attack modifiers that do damage or is about to charge, it can be pretty powerful but otherwise is usually just annoying. Cancelling a Morale result modifier for instance is kinda meh, but cancelling a Mortal Strike is pretty dang good.
Immobilize: Cancel a March or Shift action on a revealed dial. Probably the hardest to get applied to something, but can completely stall them for a turn if theyre melee (or situationally range). issue is a lot of units that get it end up dying before it matters anyway (in my experience).
Panic: One panic isnt that big of a deal, two or more can get scary as thats how you fish out those really brutal morale cards. Insane luck if its deadly or not, often i end up causing a Sev3 test and just reapplying 2 panic tokens...but then i get that one where i force the unit to Reform facing the wrong way right before they charged
My vote would go for Stun since while it isnt as effective as Immobilize, it can completely shaft your maneuver by cancelling the turn/wheel/charge/skill.
Obviously if we contribute abilities its probably one of the weakest, since nobody does anything special with stuns and like i said earlier it can still be a completely moot thing (stunned my daqan who were doing an init3 attack anyway...whoopie...).