It depends on the time period, environment, and enemy I'm fighting. Of course conventional sword and board is the iconic and pretty imagery that I immediately think of...For sheer effectiveness I'd roll with a polearm of some type. I'm not a very large and powerful guy thus trying to smash through an enemy line with sword and board would promptly get me splattered by bigger folk. I'd rather harry enemies at the end of a pike, spear, or halberd. If I absolutely had to choose a one handed weapon from the periods of 12-1500 I'd use a flanged mace. I've always had a love for them.