I voted for:
Crobat: I am actually a huge fan of the entire Crobat family, but Crobat was an absolute favorite of mine in generation 2 and I used it a lot when I first played those games. It is an incredibly fast pokemon that is surprisingly strong. I have also always liked its design with the four different wings, it makes it look really awesome and its smaller body definitely give it a much closer appearance to an actual bat. I also want to say, for Golbat specifically, I loved Golbat when I was a little kid because of its original sprite design with its giant tongue hanging out, which has sadly and mysteriously disappeared since then.
Vileplume: Vileplume is so cool and was one of my favorite grass pokemon back in the older games, I liked it's dark and mysterious appearance. I still like it now and I even placed it on a really cool sunny day team with "chlorophyll" back in my white version, and it was impressively powerful. It has a pretty decent move pool to boot.
Muk: I do like Muk quite a bit despite the fact that I have not used it to its full potential. Something about a gigantic thick pile of animated sludge has always seemed really neat to me. Plus this pokemon walls quite impressively which is what (IMO) is the best way to use poison types. I will admit, its move pool isn't all that spectacular, but it has access to all of the best poison moves which is more than enough for this guy.
Gengar: Honestly, Gengar doesn't really need much of an explanation. This pokemon has been ridiculously amazing since its original conception in generation one. Gengar has never not been a spectacular pokemon. My favorite memories of Gengar is always tied to "Pokemon Stadium", I seriously can not play that game without having a Gengar on my team.
Swalot: My absolute favorite solid Poison type pokemon is definitely Swalot. I love its bright purple coloring and I always have enjoyed its mustache. Swalot is also incredibly useful as a wall pokemon and I definitely used it in both my play through of Sapphire version and Omega Ruby and it didn't disappoint me in either games (however I think it was strictly better in OR than it was in Sapphire). The sheer bulk of this pokemon combined with a move pool that greatly supports that style, I threw Swalot in to so many battles where I just used it to waste time or KO very slowly. Throw this guy in front of a bus, he will totally be fine.
Toxicroak: Toxicroak is definitely one of my favorite aggressive Poison type pokemon which I feel are few and far between. Although Toxicroak is incredibly susceptible psychic, generation 6 has altered the weakness and resistance chart in favor of this guy. Psychic types are not the power houses that they used to be, which has given Toxicroak a lot of breathing room. Toxicroak has an incredible move pool as well as some pretty impressive stats. To top it all off, it has a very vibrant and deadly looking design with an intimidating smile, what's not to like?
