Fantasy/sci-fi ones, like Warhammer 40k’s Power weapons or a lightsaber.
Using a blade on metal is usually a bad idea. The reason is fairly simple, you’re going to damage the weapon. It doesn’t matter how sharp the blade is; in fact, sharper blades will actually suffer more damage. The edge itself will chip and deform.
Actually using a sword or dagger against someone in plate armor isn’t about cutting through the armor itself, it’s about finding the points of articulation, where plates meet, and stabbing through that gap.
The only time you’d want to be striking the plate directly is with a weapon designed to pierce armor, or just confer massive amounts of force through the armor. That’s maces, flails, war hammers (the actual weapon, not the game), mauls, and some polearms. Basically anything that’s either a glorified beatin’ people stick, or a beatin’ people stick with a spike or two on it.
I say “some polearms” because, there is a lot of variety in polearm styles, each with a different role in mind. Some were designed to pierce armor, while others are knives tied to the end of a really long stick. In a few cases, like the halberd, it’s a combination of both.
Now, if your character has an enchanted weapon that’s resistant to damage, or it’s some kind of very high tech weapon, again, like a 40k Power Sword or a lightsaber, then any conventional armor isn’t going to be much of a threat to your weapon, or even much of an obstacle.
If you’re wondering, the worst kind would probably be historical katanas. They were just not designed for contact with metal, and trying to use one against someone wearing plate or chain would quickly slag the blade.