Cool, my favorite type! It has the all Pokémon with the most personality, the most interesting, and inspire the most creativity, at least in my eyes. My favorite Poison types are Weezing (it's the first Pokémon I think of when I think "poison," and pollution monsters are amazing) and Tentacruel (Acid Spraying Jellyfish). As for the questions:
Which Generation Holds The Best Poison Types?
The first generation, hosting pollution monsters (Weezing and Muk lines), Jellyfish (Tentacool/cruel), snakes (Ekans line) and man-eating plants (Bellsprout line). The Gastly line is great as well. The most visually captivating lines come from here.
What Is Your Favorite And Why?
Weezing. It plays upon the common theme of multiple heads with mutation/fusion, morphing from Koffing into a asymmetrical being that spews pollutant gases to maintain balance in the air, looking deformed from its' own toxins, like its evolution was a mistake. Feels very Poison-type. Sentient pollution is one of my favorite concepts. Fun concept, fun Pokémon to draw, fun to battle with, just fun.
Least Favorite?
Drapion. Weezing, Muk and Garbodor convey 'pollutant mutant' clearly, things you should not be playing with, Tentacruel looks like it could poison with its' mere glare, let alone its' venom, Gengar looks like its' shadowy body is serving as a cauldron for a toxic tongue. They basically look striking, not in a 'badass' way, but in a dangerous, threatening way, like one would expect from a venomous/noxious creature. Drapion, meanwhile just looks too stupid for me to take seriously, its' gigantic arms jutting out of its head just ruins any respect I could have for its' design. I don't begrudge anyone who does like Drapion, I just find it looks exceedingly silly, and not in a way endearing to me personally.
Who Is Your Favorite Gym Leader/Elite Four Poison Type User?
Roxie, even if I haven't played BW2. Her design and concept is unique and full of personality. How many rock star trainers, let alone gym leaders, does one come across? Plus, her Pokémon, Koffing, Scolipede, Garbodor, convey similar levels of personality.
I know I used 'personality' a lot, but I really do love the members of this type, since they seem to have the best designs and concepts behind them by-and-large.