If by strongest you mean which benzodiazepine is most psychoactive by mass, then I believe the answer is Triazolam (Halcion).
Standard dosages in pills is .125mg and .25mg.
I have had both - they're quite potent.
According to wikipedia doses exceeding .5mg are unsafe... Not sure how true that is, as benzodiazipines are generally very safe by themselves, and I personally have taken more than that...
No matter, you have your answer on potency.
Now, that aside,
Alprazolam (Xanax) is the strongest commonly seen agent, and the best for combating anxiety.
Lorazepam (Ativan) is strong, but doesn't do much for my anxiety, and is definitely worse than alprazolam in terms of Antereograde amnesia. When I first started taking
Ativan, I literally lost a week of my life.
Clonazepam (Klonopin) is alot of fun, and honestly, I've only ever taken it recreationally. It's prescribed primarily as an anti-convulsant/for insomnia, I believe, and isn't widely used for anxiety anymore, as far as I know. I'm not a doctor, though, so don't take my word as gospel. Very fun benzo.
Diazepam (Valium) is the original, and in my mind really fits the bill as such. Doesn't help my anxiety at all. Lingers for a long time (half life is something like 50hrs), and honestly doesn't even give me somnelence - not to say that I have trouble sleeping on a good dose. I don't use it cause it doesn't achieve the medical goals I need - by that I mean I don't find it to be potenetly anxiolytic.
http://en.wikipedia.org/wiki/Halcion