Honestly, quite literally every pokémon is capable of being dangerous. All of them are capable of wielding forces of nature humans only dream of coming close to harnessing, and pokémon do it with the intent to battle against others. This is one of many, many reasons why we must respect all of them.
But! As that’s hardly a proper answer, I must say it’s difficult to decide. Perhaps the legendaries, as those are frequently the avatars of the strongest or most fundamental elements of nature themselves. How can you possibly defend yourself if the avatar of space decides to thrust you into a trans-dimensional rift that leads to the other end of the universe?
Out of the more terrestrial pokémon, though, I suppose the answer would indeed be poison-types … or, more specifically, the likes of grimer, muk, koffing—all the ones that possess abilities that can kill or corrode on contact. While we have antidotes that can handle the vast majority of poison-type abilities, the bodies of grimer and muk contain bacteria that, if worked into open wounds, could infect one with a whole slew of rather nasty incurable diseases, some of which are highly infectious if given the right vectors. Then you have koffing and weezing, whose gases suffocate, rather than poison, which is not something that you can treat unless you’re aware enough of it happening to get as far away as possible. Unfortunately, some of the gases expelled by koffing and weezing distribute themselves rather quickly, so the range of effect can be quite wide and inescapable. The gastly line operates on the same principle, actually, only with an additional soul-stealing element due to their ghost-type natures.
The other pokémon, while fearsome, are at least avoidable for the most part.