Your right, there is no time period restrictions. I am just wanting to see your preference, however I guess I should make it to were it needs to be a weapon you KNOW how to use.
OK, I'm going to assume man portable individual weapon here:
Pregunpowder? Longbow. I'm not bad with one. It was not surpassed as a mankiller till after rifles came about, has a good range and rate of fire, and multiple ammo options. Wearing a byrnie, CoP, greaves, Bacinet + aventail, and vambraces. With a sword, dagger, & buckler to back me.
Modern? AK-47 & bayonet. I'm decent with one. The round hits hard, the range is not longer than I can make use of, and the weapon is reliable as all getout. With a good armoured jacket, helm, and other 1st world battlefield kit. and a S&W XD .45ACP for backup.