I am guessing Fentanyl? (but I may not be correct because fentanyl has a much higher therapeutic index)....
Fentanyl is 80-100x more potent than Morphine (40-50x more potent than heroin); but has a therapeutic index of 270 (whereas heroin has a therapeutic index of ~69). I think with the potency of a chemical like fentanyl you are far more likely to accidentally OD though.
EDIT:
Sufentanil (5-7x potency of Fentanyl; 200-350x potency of heroin; 400-700x potency of morphine)
Therapeutic Index: 48,076
Carfentanil (100x potency of Fentanyl; 5,000x potency of heroin; 10,000x potency of morphine)
Therapeutic Index: 10,600
3R,4S,βS-ohmefentanyl (28x potency of Fentanyl; 1,120-2,800x potency of heroin; 2,240-5,600x potency of morphine)
4"-fluoro-3R,4S,βS-ohmefentanyl (9,000x heroin; 18,000x morphine)
These are just opioids.. I am sure I can find you some other dangerous substances.