A first pass answer is 662-4342.
The Therapeutic index is calculated from rat tolerances; LD50 is a rare number to see for human beings(!). The toxic dose of a compound can vary widely over a class of compounds as well as does the Effective Dose (ED) midpoint (ED50).
But I found this book reference:
Benzodiazepines have a wide margin of safety based on experimental studies in animals. In the rat for example, the lethal dose of alprazolam is 331-2171 mg/kg. The effective therapeutic dose for alprazolam is 0.5 mg/kg which results in a therapeutic ratio of ... 662-4342.
The page goes on to say, at rather great length, that human evidence is that despite the fact that this is a popular class of drugs to try to kill yourself with, there are relatively few human deaths, which is pretty ultimate a judge for toxicity, though you could look for FDA warnings on the class of drugs.
I would bet that you can find relatively toxic benzodiazepines, but for the ones that pass screening they seem pretty benign as a class. There are other issues with these drugs rather than mere toxicity - they are quite addictive, so therapeutic index is not an ultimate judge of the adverse effects of a drug.
BTW taken literally, this is a hard question you ask. If you are expecting a compiled list of all such drugs, cross referenced to their Therapeutic index, that is really more like the work for a paper rather than 25 points of stackexchange karma IMHO. On the other hand maybe someone has this information at their fingertips...