In each game please
P.S. Don't just go Saying Magikarp. Just No
For eg.- Litwick in the first few floors of Celestial Tower in Black 2, and Unown in the Ruins.of Alph are both Pokemon, who are found at a 100% rate.
There are many such examples, see more examples [here.][1]
Hope I helped! [1]: http://pokemondb.net/pokebase/203837/places-black-pokemons-100%-100%-wild-encounter-repel-tricks