The ubiquitous .38 spl. S&W Model 10/15 for a one and one only revolver choice myself.
Or a S&W 17/18 in .22lr.
Either or any of the above would do me well if limited to an "Only One" choice.
A free people ought not only to be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government." - George Washington, January 8, 1790, First State of the Union Address