The issue doesn't lend itself to a simple answer.
As you've found out, effective parking controls are essential while you have selfish, idle and cheapskate drivers who have no regard for other people's property and the direct, and often indirect, effects on others of their actions. Ironically, the issue of paying for parking has a corollary: what are people prepared to pay for protecting their parking rights?
So a necessary 'evil'?
DWMB2 put it: but the contents of the contract between the landowner and the PPC is probably more important than which PPC it is. We've seen plenty of cases on here and PePiPoo where the tail was wagging the dog, with the landowner unable to get charges cancelled, for example. In any consultation response, you might be keen to recommend that any contracts they sign give them good leeway to get charges cancelled.
So..:
What would be the parameters of the contract e.g. the PPC is paid by the residents to protect their interests or you expect them to do this FOC etc? How many residents? Over what 'customer' base would direct contract costs effectively be charged? IMO, cancelling parking charges is not your first consideration. We're all expectancy theorists(ETs) at heart so, would each resident be prepared to pay £X per year to ensure their rights are protected or run with the risk that they could pay less but with the possibility that their forgetfulness might cost them £Y in parking charges?
Talking of which, the displaced drivers are also ETs, so you would have to think about what level of parking charge would deter? And not just the charge, it's the likelihood of receiving a charge which also feeds into the ET matrix. And what is the problem e.g. long or short-term (commuter parking or parking for shops) because this as much as anything else sets the scene.
And you could ask the other sites why they chose who they did, why and problems encountered.
So, the issue isn't defined by 'least worst PPC'.