Must we stick only to JRPGs?
The Ultima series has the most impractical weapon, but it's intentionally designed to be that way. The Glass Swords can kill any killable character or creature with a single blow, but will shatter on impact. They're basically disposable death. Now, I imagine that we are, at least to some extent, applying realistic contexts to these weapons, so if you consider that a glass sword is weak enough to shatter on impact, you gotta imagine that it needs to be handled pretty delicately in the time that you're carrying it around and not using it. When you're on a quest of epic adventure, it would suck to have to tread lightly so that you don't break a disposable weapon.
A close second goes to Ultima's poison dagger, though this is strictly in the context of the game. Sure, in a real life context, a poison dagger is, by definition, pretty damn lethal, quick, silent and concealable, but in the Ultima games, there is no stealth. There is only combat, and the effects of the poison are moot on enemies, since the effects are too slow to be of any assistance. You'll end up killing them based on the dagger's base damage well before the poison effects will even come to play. And, in a video game, knowing that you've caused a troll long-term illness is of very little consolation.
In terms of JRPGs, though...
Now, before I get the lectures: I say this *jokingly*. I admit I never played the game before, so I emphasize that I'm being facetious when I say that Persona 3 doesn't seem practical to me, in that your weapon is that you SHOOT YOURSELF IN THE HEAD. LOL
