The ones I use the most are my 6" x 24", my 6-1/2" square, and my big square one (think it may be 16"). Any others I have are ones that pertain to special shapes, like 60• triangle, set of semicircles, etc. that come in handy for specific uses.
I would like to get the next size up from the 6"x24" for cutting wider fabric. Not an essential but I would find it helpful and would use it regularly if I had it.