1954 Topps is the worst. Dead-centered copies in that set make up less than 1% of the print run.
1952 Topps isn't far behind with around 2-5% centered copies, although this set varies more by card.
1975 Topps is a mess. I'd probably put this set 3rd, but there are enough of them that you can still find centered copies if you're patient. The worst part about this set though is the tilt and miscuts.
1955 & 1956 are pretty tough as well. 1953 is surprisingly not that bad.
Here's one of my favorite 1975 examples I've come across lately... Way to go PSA!
