The Pokemon that are definitely non-legendary (I'm not sure if I should add Manaphy and Heatran and all them, but these Pokemon are definitely not legendary) that have the lowest catch rate of 3 are the entire Beldum Evolution Family.
Skarmory is still pretty close though, Volcarona (legendary?) has a catch rate of 15, but then many other Pokemon including Skarmory have a catch rate of 25.
Hope I helped. :)