Koffing is my favorite poison type by far! He's so cute and happy and he's my favorite color, purple!
He's even my icon on Tumblr!
As for dual types, I tend to like ones that combine it with Grass. Probably Vileplume (because it's based on rafflesia, which are pretty weird and cool) or one of the Roselia family, because I'm biased when it comes to roses for obvious reasons.