I would assume that the least effective benzo for SA would be the the least potent with longest release in the benzo class. Benzos with long release times don’t give you the near-instantaneous calm “spike” that you get with high potency short acting benzos like Xanax and Ativan. With that in mind, the least effective benzos would probably be Valium (diazepam) and Librium (chlorodiazepoxide). I’ve personally had no luck with benzos whatsoever, after trying Xanax, clonazepam and Ativan. My doctor put me on 4mg of Clonazepam per day and I felt nothing, not even sleepy. I guess benzos just act differently for different people.
"I split my mind ten thousand times, but in every world there's no exit... I waited on the tracks of reason, but my train of thought, it never came." - Silent Planet