Poison's for people without the fortitude to smash their head against a wall enough to kill anything.
Although, I suppose a paralyze or damage health would be the best poison.
Ummm? Or not wasting a thousand arrows. Who wudda thunk not everybody uses hammers to solve problems
As for the original poster. This website will give you some answers http://www.uesp.net/wiki/Oblivion:Poison. But yes. High damage health or a paralyze potion is probably best. Depends on what you want from a situation. Drain fatigue is not all that bad either. For example. You want to land an arrow and go in melee. They will do reduce damage with no fatigue.
If you are looking for a more specific answer maybe I could help 8).