If you, for whatever reason, choose to have a single stone, I'd suggest a 1K. This may have already been recommended to death, but the reasoning is sound. A 1K can create a perfectly serviceable, if not impeccably polished, edge, and can restore a fairly dull edge, although you may have to work at it for quite some time.
One of the main reasons for having an assortment of stones is to save time- you can spend just a few minutes or less at each stage and quickly go from dull as can be to good enough to use.
Director of the B&B Stjynnkii Membörd Dummpsjterd.
Baby Brain Smooth.
B&B Old Fogies Club and the original Old Fogie