Woo! starting this up again, I see! =D
Poison-types are hard to judge. They usually have a dual-type, but even then, they usually act more as their secondary typing, rather than an actual Poison-type. These poor souls just don't really know what they are yet.
Gen IV provided Toxic Spikes, Poison Jab, and Cross Poison, so these underrated Pokemon have a little more identity, but "little is an understatement.
Poison-types are the Physical-based counterpart to the Grass-type, having slightly weak moves, but having a variety of tactical and Status moves.
However, even though the rules say that dual-typing is allowed, I think that it's an exception for Poison-types, because of their lack of independence as a type. Therefore, I will only name the best mono-type (Not trying to start a revolution or anything. Just an honest, self-opinion xD).
I definitely pick Muk as my choice.
As disturbing as its name really is, I've got to give props for such perfectly distributed stats. It can wall, it can sweep, it can stall, and lots of other things. Think of it as a Claydol counterpart.
It's got an enormous movepool, being able to use Thunder AND Fire Blast, making it perfect for a Mixed Weather team.
With Explosion, it can also make a good Suicide Lead.
With its Stealth movepool, containing Mud-Slap, Minimize / Double Team, Toxic, and Focus Punch, it can sweep half a team with the right moves being made (Must I bring up Gen I's Minimize-ing Koga epidemic? :P)
It can utilize elemental punches and Shadow Sneak for a versatile killer set, it can Curse up and be a Physical Tank; I could go on forever. There's no end to what this beast can do, and definitely deserves the top prize. It may be a Poison-type, but it basically act as whatever type you need it to.
Now for some runner-ups!
Unlike Arbok, Weezing has the impressive stats to be strong without setup. However, also unlike Arbok, it lacks versatility, and needs specific movesets to be viable. Otherwise, it just gets eaten alive.
It usually works well as a Crippler / Physical Wall with Will-O-Wisp and Spite; works as a Suicide Lead with Explosion and an immunity to Spikes.
Always loved it, but it was too weak. Because of Gen IV though, it got a MASSIVE boost (As did all Poison-types!).
Not great stats, but it's movepool and abilities are perfect! Arbok is the only Pokemon I've used that benefits from BOTH of its abilities (I swear, I feel torn over deciding whether Intimidate or Shed Skin s better...). With Intimidate, Arbok makes a great Physical Tank, with an illusion of better Defense (About a Base 78 with technical multipliers, etc).
Also, with Shed Skin, it can tank with Rest, because of the 50% higher chance of waking up. With all luck accounted for, this makes the most efficient Reliable Recovery in Pokemon.
Pair that with Poison Fang for Bad poisoning or Gunk Shot / Poison Jab for raw power, Crunch and Ice Fang for coverage, and Screech to make for Phenomenal sweeping, and you've got a massively overlooked Sweeper.