Realistically, you're not going to find an authentic historic Katana anywhere outside of Japan. They're considered national treasures and are illegal to export. So anything you get in Cataclysm is going to be a replica of some sort anyway. At best, you'd get a WWII era sword that was captured by a soldier and brought back.
And any sword is probably going to be pretty good for hacking up zombies, assuming it's decently made and not just a wall-hanger. Swords are pretty much ideal for cutting, and one of the few items that had no other purpose than killing people.
For pure slashing ability, you'll want a curved blade. Katana, Sabre, Scimitar, and the like are all good for that. Straight bladed weapons aren't quite as good, but since your goal when hitting a zombie is to hack them apart and not just make them bleed, it's not a big deal. Remember that a human when slashed will probably fall down in shock and grab the cut. Doesn't have to be too deep to take a person out of commission. A zombie doesn't care, so it has to be a serious blow to make them notice or disable them. A short hacking blade like a
Falchion might actually be better than a katana, actually, since those were designed for close quarters work and hacking through both armored and unarmored opponents as well as wood (pike hafts, specifically).
Thinking about it, a
Boar Spear and a falchion would be a pretty good combination. Stick a zombie with the spear to keep it from advancing, and use the sword to dispatch it or fight off any other zombies. A shield would also be quite handy, since your basic zombies can't really do much against a metal shield.