Gengar is the best Poison type in DP. You don't even have to trade for one if you own a Gen III cartridge. All you need to do is insert the Gen III cart, then go to Old Chateau and go to the room with the red eyes. There, you have a 4% chance of finding a level 16 Gengar. This can be obtained as soon as you beat the second Gym, and with Gengar's incredible stats, you have a huge advantage early game. And it remains incredible later in the game, what with its OU worthy offenses and fairly large movepool. Levitate is also a great ability.
If you don't have any of the Gen III games, Toxicroak is a good alternative, at least if you don't plan to use better Fighting type Pokemon. It has a nice base 106 Attack and a decent enough movepool to use. It's typing is pretty good, as it gets a lot of cool resists. Although it wouldn't be taking hits in competitive, ingame it can actually make use of those resists, as your opponents lack EV training and good IVs. Also has a Water immunity with Dry Skin.
The best Dark type available has gotta be Drapion (who also happens to have the Poison typing). Skorupi can be obtained at a decent point in the game. It evolves pretty late though and is somewhat rare, which is why I recommend Gengar and Toxicroak over it. It also doesn't have as good of offenses. However, it has a great movepool, with access to Earthquake, the elemental fangs, and good STAB options.
Other Poison types:
Crobat-Use Staraptor as your Flying type. Staraptor is pretty much the best Pokemon ever for ingame.
Roserade-It's a great Pokemon if you didn't pick Turtwig. I'd give it a shot if you already have a good Fighting type and are unable to get Gengar.
Haunter-If you are unable to get Gengar, that doesn't mean Haunter isn't an option. It's got decent offense and a nice movepool and typing. I wouldn't suggest using it though.
Tentacruel-While a weak movepool lets it down, it can be used if you aren't using the other Water types available. I would suggest using the other Water types though, as Tentacruel doesn't have the best offenses.
Other Dark types:
Weavile-The lack of a move tutor means that Weavile doesn't get a good Ice type attack. Combine that with the point at which you get it ingame, and Weavile is a no-no.
Spritomb-Not at all worth the work it takes to get it.