I hate that question. Most dangerous?
- They are all dangerous - all chemicals can kill you. As such, all chemicals need to be treated with respect.
- "Danger" has many aspects - toxicity, explosiveness, reactivity with the general lab environment, ability to cause long-term health issues,... How can these aspects be ranked relative to one another? That makes for good debate over a couple of beers, but it's not a serious safety discussion.
- The question is begging for bravado: "Yeah, well I worked with X and that's twice as bad as your Y". Again reinforcing the concept that only certain chemicals need concern us.