Since I took fencing lessons for a few years when I was younger, I'd say a rapier. It can be used for either fast attacks and efficient parries, taking advantage of your opponent strenght to divert his weapon and hit him all at the same time. In fact, when you're enough skilled, it becomes like a natural extension of your arm.
But I guess bringing one in a medieval battle would be completely useless, due to the lack of the space needed to use it properly. I'd go with the standard, reliable longsword (possibily double-edged) in combination with a shield. The sword is the only weapon you can use effectively to parry attacks and having it combined with a shield makes you really hard to hit. I've always found the +1 bonus to the armour save when using hand weapon and shield a very realistic rule in warhammer.
I remember trying to hit a guy at the medieval-fancing lessons that was using longsword and shield, and it was really wearing, even using a two-handed sword!