Here is the rundown on Benzodiazepines.
Xanax is the strongest and the most addicting of the three since it's the only one that gives you a euphoric type of feeling. However, most people only feel effects for 2-4 hours.
Klonopin ranks second in potency, and I say that because it doesn't really give you the happy buzz that Xanax does, but it calms you down and relaxes just as good in my opinion. It also lasts a lot longer; at least 6-8 hours.
Next comes Ativan, which never really made me feel much of anything. It didn't even reduce any physical symptoms either like palm sweat and fidgeting, both of which Xanax and Klonopin help a lot for me.
As for Valium, I've never been prescribed it so I have limited experience with it (5 or 6 times), but it had very little effect on me and I would say the experience seemed a lot like Ativan. Valium, per mg, is also much weaker than the other three benzos I've mentioned here. Xanax, Klonopin, and Ativan have starting dosages of .5 mg, while Valium's starting dose is 2 mg.
Hope I helped.
Peace