All the stereotyped hard drugs because most of them live up their hype and I have experienced what they can do to you having lived through various difficult periods in my life where I met people who had fallen into the trap of using them. And it never ended well, hence why they were living in homeless hostels and their entire day was based around getting money for drugs to then take drugs, or drink, or both.
I wouldnt mind the stuff that has the least addicting potential and least harms. I think an education and healthy culture around their use is key. I was fortunate growing up that I had access to circles where drugs were taken responsibly and the care of both the individual and the group were center stage. Where values, expectations, intentions, limits, principles, standards, call them what you will, were etched firmly into the atmosphere. There was a great emphasis on responsibility and on maintaining social, emotional, mental, physical and spiritual health - primarily because these circles were surrounded by lifestyle choices that meant you lived up to being able to get up and do the things you wanted to do in the first place. Otherwise, you got left behind, so to speak and the environment drowned you out. Social responsibility was key.
I think the biggest problem is when the journey into experimenting with drugs involves lots of unnecessary risks and when you are facing unfamiliar environments, situations, people etc. In the process of escaping to fulfill your drug use/abuse you end up running away from the support network and the protective shield of a well established bubble, routine, atmosphere etc.
I wouldnt want my kids to end up going down that road because then the need to continue to take more risks grows increases endlessly and grows until there is nothing stopping you from doing anything and while that sounds blissful, especially as a child who might be rebelling or going through the difficulty of growing from a child to an adult, you also lose your inhibitions and allow things into your life you wouldnt usually accept. You need grounding and your environment should be able to facilitate your growth, development and experimentation while not forcing you to take your choices "underground".