What is the most dangerous potion in Harry Potter?
I agree that poly juice potion is dangerous, but the most dangerous potion definitely is the love potion, or more preciously, amortentia.
Other potions are most-likely to change you outside or, in the case of veritasirum, make you tell truth. Love potion is trans. With it, you lose control over yourself. I would say that it's similar to imperius curse.
Love potions are something like sexually abusing/rape in the muggle world, but instead of the person defending herself/himself, you gave them drugs. The point of a love potion is making someone who isn't interested in fell in love with you.
The only idea of it is terrible, we saw Ron under Romilda’s potion. The only thing we knew is that he was ‘in love’ with her. But things can end up a lot worse. The fate of Tom Riddle Sr. is horrible, one of the worst in the series. He was under potion for a long time, She basically raped muggles, but in the wizarding world, love potions are legal. Weasley twins had potions in their store and probably a lot more people all around the world.
Now, if a kid is made under influence of a love potion, he or she can't love. You aren't only destroying the life of the person you raped, you are destroying the life of someone who was supposed to have a choice. We know one person who was made under influence of this potion, and it didn't turn out well.
I could write more about this, but the point is clear. They are *probably* most dangerous potions in the wizarding world.