For those saying gnosticism, I'm curious at to what you think that term means. As generally used it is an umbrella term that includes so many groups, practices and beliefs, over such a long period of time, and in so many different places, that it becomes largely meaningless in discussions like this. Which particular gnostics or beliefs? After all, most gnostics weren't Christian heretics, but rather pagan syncretists who tried to take elements of various religions and philosophies and merge them.
I'm not sure that I take 'dangerous' and 'damaging' as being the same thing, as I would understand the first term as having more to do with potential + actual harm, while the latter I'd see as being mostly just about actual harm. But anyway, if I had to choose one that fit either/both, I would say Arianism. It lasted in the east for at least 60+ years, and in the fringes or outside the empire for hundreds of years. It was held to by many emperors, who persecuted and martyred Christians. I think it was the first major heresy Christianity faced on a higher intellectual level, and which required greater theological minds to refute. It was also dangerous because at the time it was not only that 'the world awoke to find itself Arian,' but even within the orthodox faithful there were disputes and divisions (Constantinople and Rome not in communion, Alexandria and Meletian Antioch not in communion, etc.) Of course you can say, rightly, that the Church was never truly in danger (Matt. 16:18)... yet it must have seemed pretty grim at times.