I don't know which are the safest but I can certainly will tell you which ones are the worst (IMO & IME) in terms of cognition and long term organ damage.
The absolute worst especially for cognitive function & for long term organ damage:
1. Lorazepam (with small to medium tolerance, any dose above 2mg+ will provide extreme amnesia). I've tried more benzos than I care to admit and I've never encountered another benzo that produces this much amnesia at a medicinal dose.
Something to look out for is the class of benzodiazepines called 'nitrobenzodiazepines' which are carcinogenic and much more dangerous than other benzos. I've been taking 4mg/day clonazepam, without skipping a day for the last 12years, ever since 2004!
Below is a direct quote from a very reliable source that I saved and if pestered with comments to give the reference, although I don't know it, it shouldn't be a problem finding it...in case there are skeptics
"Nitrobenzodiazepines such as nitrazepam, nimetazepam, flunitrazepam, and clonazepam are more toxic to the liver than other benzodiazepines as they are metabolically activated by CYP3A4 which can result in cytotoxicity. This activation can lead to the generation of free radicals and oxidation of thiol, as well as covalent binding with endogenous macromolecules; this results, then, in oxidation of cellular components or inhibition of normal cellular function. Metabolism of a nontoxic drug to reactive metabolites has been causally connected with a variety of adverse reactions. Nitrazepam has been associated with severe hepatic disorders, similar to other nitrobenzodiazepines. Clonazepam is a chlorinated derivative of nitrazepam and therefore a chloro-nitrobenzodiazepine."
I hope this helped!
BTW, Keif' the reason it is said benzos are 'healthier' is because of their therapeutic index, and that reason alone!!! More dangerous drugs like barbiturates, or worse (methaqualone) which have a very narrow therapeutic index meaning that their medicinal dose is very close to their lethal dose/overdose. The comment made by Keif' can be very mislead as far as harm reduction goes! Benzos are still physically and mentally addictive and produce both physical and psychological withdraws. With time benzos not only destroy the body's organs but significantly interfere with the brains day to day operations such as memory or cognitive performance. Barbiturates on the other hand have never gotten in the way of memory or learning, even though their therapeutic index is much narrower. Therefore, Keif' no, benzos are not healthier!