"Easy" rarity Pokemon are the easiest to hunt, as their pairs
produce eggs a lot quicker.
These are the rarities from easiest to hardest: (not including
Ditto or Legendary)
Easy
Medium
Hard
Special
Starter
Rare
Event
Unown (really hard)
Overall. I'd say the easiest Pokemon is magikarp. Low ehp and gem
cost makes it fairly easy to hatch and obtain a bunch of eggs in a
short amount of time.
Really when it comes to the daycare it just kinda boils down to
luck. You can do things to help speed it along but that's about it.
Some days you can get 20 eggs and others you're stuck with like
ten.