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.
Wenn ist das Nunstück git und Slotermeyer? Ja! Beiherhund das Oder die Flipperwaldt gersput!