the 45 is the most common-used the most often for a couple layers of fabric-
the 60 is good for flannels, thicker fabrics, more layers
and the small ones (28 & 18 i think-)are good for small pieces/shapes-going around curves, tight place, around templates, ect.
some people just like using the larger blade over the 45- some people hate using the large one and only do so when the project makes it necessary . the little ones are kind of project dependant- but come in handy - they are easy to go around templates with....more precise cutting- but not something you want to use for cutting yardage/large pieces