Personally, centering is the most important condition requirement. I will take a soft corner before taking a Wacky Package that is off centered. Here is a list of stickers that I have found to be the most difficult to find centered: Paul Maul, 8 Lives, Sweat Hard, Cheapios, Choke Wagon, Botch Tape, Bum Chex, Captain Crud, Vile, Koduck, Tijuana Smells, Secrets, Truant, Alpain, Clammy and Heartburn. I am sure that I am missing a few so let me know what you have come across.