Hi,
Normally standard cells would have buffers, INV, AND, OR, NAND, AOI (AND-OR-INV), MUX, flip-flops, etc, etc.
Are you creating your own library from scratch? If you have a bunch of cells lying around (now how does that happen?) & don't know how to group them, I think you should put everything according to technology.
If you are trying to do some admin work whether on paper or on a database, well, there's no fixed definition of what has to be in a standard cell library - AFAIK. Just look at the types of standard cells from any foundry & you'll see there are loads you can find in one alone.
But if you have some sort of vlsi assignment to do I'll say the ones I mention would be ok for a start... and good luck finishing them.