My absolute fave of a Shiny Poison-type would be Scolipede, I love the red body with teal colored circles on it. Whirlipede's shiny form is cool too due to the teal circles as well.
Also, I'd like to give Amoonguss a mention - very interesting colors!
I also like the coloring of Shiny Koffing and Weezing:
I also like the blue gas on Shiny Gastly: