I started playing a thief and i love sneaking with bows.. what ingredients can I use to make some good poisons?

Here's a http://www.uesp.net/wiki/Skyrim:Ingredients. (be warned though, I consider this a large spoiler as there exists a perk solely for the purpose of improving your ability to discover the effects of ingredients)

Some of the most useful effects I can think of for an assassin's poison are Slow, Paralysis, and Damage Magicka/Stamina Regen (easy to reach 100%).

What's the use of damage stamina regen though? Do they fall down or something?

Damaging an enemy's stamina regen is definitely less useful than damaging magicka regen, but it prevents melee enemies from using power attacks and, from what I've heard, shouts. Essentially, it results in you taking much less damage. One could look at it as fulfilling the same purpose as increasing armor rating.