There's one that's unique and is part of a guild quest, so I won't say which. The plant itself is Jarrin Root. 2nd in line is Crimson Nirnroot, which is only found in Blackreach, which is part of the main questline. I think you can go there whenever ya want though. That's for making them anyway. Pre-made ones are Deadly Poison for 65 points of damage, and the Deadly Paralysis Poison for 15 seconds of paralysis. Basically "Deadly"'s the top prefix.
if you want one that works effectively and is relatively easy to use as a stealthy character, you want imp stool, human flesh, and slaughterfish eggs. it creates a paralyzing potion that deals a burst then leaves a lingering poison damage effect.
there's also one that's more combat oriented, deathbell, giant lichen, skeever tail. does weaker damage in only a burst, but makes the target weaker to poison at the same time.
i've done some tests with the alchemy/enchanting/smithing exploit, plus a mod that adds earrings (all enchantable, and all can be worn at once and use head slot enchantments for a total of 12 pieces), and the alchemy revisited mod, 56% better potions per piece, if i make the first poison i said, 331 burst poison, 166/dps for 10 seconds lingering, and paralyzed for the entire duration.
mods used http://skyrim.nexusmods.com/mods/8624 http://skyrim.nexusmods.com/mods/15918
Pretty pretty is the horsey who hath drunken the magic tequila