X-acto has many different blade types and knives, but my favorite and all I ever need are the #11 blades and the knives that match.
I primarily use just two tools. One has a nice comfort grip and full size, the other I cut off the end making it an excellent tool for scoring RC bodies for easy cutting of the lexan.
I recommend keeping plenty of these blades around and replace them often. A dull x-acto can be very frustrating. I replace mine for every lid I do or important cut I have.