[Flyndaran]

TV shows are obviously heavily limited by budget and having to use humans in make-up.
Books and RPGs can have all sorts of off the wall aliens.
I always wanted an otherwise boring species that needs to wear a space suit because their biology/atmosphere would be explosive/toxic in human comfortable environments.
Star Trek had Benzites that constantly spewed benzene to breathe... with no mention of how horrendously carcinogenic that gas is.

I like the few members of a primitive not yet officially contacted species.