In order to have the best possible breeding pair, your pokemon
should have the same pokedex number (a magikarp with a magikarp
would be better than a magikarp with a gyarados) and have different
Original Trainers (OT). If it's not possible because of mono-gender
pokemon like chansey, you'll have to check the Egg
Group Guide.
For more information about breeding in general, you can check the
FAQ