What is the longest route to hatch eggs?

Solution 1:

Lumiose City Centrico Plaza

This plaza surrounds the Lumiose City Gym in a circle. If you hold left or right on that screen you can go forever.

Solution 2:

Route 7, which is the path right in front of the Daycare is very long at 220 steps, straight and has no obstacles or long grass.

The Daycare is roughly in the middle, and the path from it to either end and back is long enough for eggs to have a chance of spawning (which requires at least 255 steps).

Combined with the easy access to the Daycare to get new eggs, this may be the best place to hatch an egg in Pokémon X and Y.

With a Flame Body Pokémon combined with the Hatching O-power you should be hatching in no time.