There are bad contracts, and there are regrettable ones. Usually those go hand-in-hand but not always.
When compiling the list of "regrettable" contracts, I take into consideration the terms of the contract (with respect to length and price), the age of the player, the player's production (both prior and post signing), the injuries sustained, the games played as well as other minor tangibles. That sounds like a good set of criteria on compiling a list of worst contracts.
Whether or not you agree with the list of contracts I came up with, you'll see that what seems to be the consensus for worst contracts is not always the most regrettable. A quick example is Brian Campbell's contract. Though many believe he is an overpaid underachiever, does he really have the most regrettable contract on the Florida Panthers?
It's not always the highest paid player that has the most regrettable contract.