I know the A-series is more common, but I sometimes prefer the B-series. It's nice if, for example, you're looking for something between A4 and A5 (i.e. B5).
Individual images made by Wikipedia user Bromskloss (CC BY-SA 3.0)
It's the same, the difference is the starting "0" size. For the A-series, the area is 1 m^2. For the B-series, the shorter side length is 1 m. The C-series is the geometric mean of the areas of the A- and B-series.
When I was a student, all my notebooks were either A4 or B5, these days even A5 is too big for me and I end up with A6 for most of my handwriting stuff.
Never seen a C-series paper though, didn't even know it existed.