My personal favorites are the Zweihander or a khanda for a large sword. For a one handed all purpose blade I'd pick a Kriegsmesser that was made from Damascus Steel.
Twin katanas with a crackling energy field around the blade that when set to the highest setting, is a monomolecular cutting edge that can go through almost anything, or the energy can be turned down to a simple blunt edge.
It looks massive and heavy yet relatively short, the spikes of the guard may inflict damage to the wielder him/herself in worst case, and the barbs at the point seem like they could get stuck on a lot of things, though they seem as they might hurt when you get struck by those^^. Though it looks shiney and assumingly unbreakable, I wouldn't choose that one. And any sword that is unbreakable is cool
Apart from the Oblivion sword I mentioned and the nice dagger Freakenstein showed, I'd say lightsabers are cool because they're practically small when in rest, and can cut through a lot of things. Though I'd fear self-inflicted damage as long as I'm unfamiliar with them, they're pro-only.
Sorry everyone, but I'm going to have to say that Link's Master Sword takes the cake for this one. It can emit powerful blasts of energy and repel the darkest of evils. Try beating that with your everyday sword.