I'm not a doctor but I would say that Halcion is the strongest. .25 mg of Halcion is equal to .5 mg of Xanax (or Klonopin) or 1 mg. of Ativan or 10 mg. of Valium.

Ativan, Xanax, Klonipin and Valium are primarily anxiolytics used in the treatment of anxiety where as Halcion is primarily a hypnotic designed to induce sleep and/or treat insomnia.

They all have different half-lifes (how long before half of the effects of the medication go away):
Xanax 6-12 hours
Klonipin 18-50 hours
Halcion 2 hours
Valium 20-100 hours
Ativan 10-20 hours

They also take varying times to "kick in". Halcion and Xanax are the fastest acting (30 minutes or less). The rest take 1 hour or more.

Hope this is helpful.